/* ========================================
   CSS Variables & Themes
======================================== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    /* Purple Theme (Default) */
    --primary-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --secondary-gradient: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    --accent-gradient: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);
    --bg-primary: #667eea;
    --bg-secondary: #764ba2;
    --calculator-bg: rgba(255, 255, 255, 0.98);
    --display-bg: linear-gradient(135deg, #1e3c72 0%, #2a5298 100%);
    --btn-num-bg: #ffffff;
    --btn-num-color: #2d3436;
    --btn-op-bg: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --btn-op-color: #ffffff;
    --btn-func-bg: #f1f2f6;
    --btn-func-color: #2d3436;
    --btn-equals-bg: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    --btn-sci-bg: linear-gradient(135deg, #a8edea 0%, #fed6e3 100%);
    --btn-sci-color: #2d3436;
    --text-primary: #2d3436;
    --text-secondary: #636e72;
    --text-light: #ffffff;
    --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.08);
    --shadow-md: 0 8px 24px rgba(0, 0, 0, 0.12);
    --shadow-lg: 0 20px 60px rgba(0, 0, 0, 0.15);
    --shadow-xl: 0 30px 80px rgba(0, 0, 0, 0.2);
    --border-radius-sm: 8px;
    --border-radius-md: 14px;
    --border-radius-lg: 24px;
    --history-bg: #f8f9fa;
    --history-item-hover: #e9ecef;
    --panel-bg: rgba(255, 255, 255, 0.98);
    --overlay-bg: rgba(0, 0, 0, 0.5);
}

[data-theme="ocean"] {
    --primary-gradient: linear-gradient(135deg, #2193b0 0%, #6dd5ed 100%);
    --secondary-gradient: linear-gradient(135deg, #00c6fb 0%, #005bea 100%);
    --accent-gradient: linear-gradient(135deg, #5ee7df 0%, #b490ca 100%);
    --bg-primary: #2193b0;
    --bg-secondary: #6dd5ed;
    --btn-op-bg: linear-gradient(135deg, #2193b0 0%, #6dd5ed 100%);
    --btn-equals-bg: linear-gradient(135deg, #00c6fb 0%, #005bea 100%);
}

[data-theme="sunset"] {
    --primary-gradient: linear-gradient(135deg, #f093fb 0%, #f6d365 100%);
    --secondary-gradient: linear-gradient(135deg, #ff9a9e 0%, #fecfef 100%);
    --accent-gradient: linear-gradient(135deg, #fa709a 0%, #fee140 100%);
    --bg-primary: #f093fb;
    --bg-secondary: #f6d365;
    --btn-op-bg: linear-gradient(135deg, #f093fb 0%, #f6d365 100%);
    --btn-equals-bg: linear-gradient(135deg, #ff9a9e 0%, #fecfef 100%);
}

[data-theme="forest"] {
    --primary-gradient: linear-gradient(135deg, #11998e 0%, #38ef7d 100%);
    --secondary-gradient: linear-gradient(135deg, #56ab2f 0%, #a8e063 100%);
    --accent-gradient: linear-gradient(135deg, #56ab2f 0%, #a8e063 100%);
    --bg-primary: #11998e;
    --bg-secondary: #38ef7d;
    --btn-op-bg: linear-gradient(135deg, #11998e 0%, #38ef7d 100%);
    --btn-equals-bg: linear-gradient(135deg, #56ab2f 0%, #a8e063 100%);
}

[data-theme="dark"] {
    --primary-gradient: linear-gradient(135deg, #232526 0%, #414345 100%);
    --secondary-gradient: linear-gradient(135deg, #0f0f0f 0%, #232526 100%);
    --accent-gradient: linear-gradient(135deg, #434343 0%, #000000 100%);
    --bg-primary: #232526;
    --bg-secondary: #414345;
    --calculator-bg: rgba(30, 30, 30, 0.98);
    --panel-bg: rgba(30, 30, 30, 0.98);
    --display-bg: linear-gradient(135deg, #1a1a2e 0%, #16213e 100%);
    --btn-num-bg: #2d3436;
    --btn-num-color: #ffffff;
    --btn-op-bg: linear-gradient(135deg, #6c5ce7 0%, #a29bfe 100%);
    --btn-func-bg: #3d3d3d;
    --btn-func-color: #ffffff;
    --text-primary: #ffffff;
    --text-secondary: #b2bec3;
    --history-bg: #2d2d2d;
    --history-item-hover: #3d3d3d;
}

/* ========================================
   Base Styles
======================================== */
body {
    font-family: 'Inter', sans-serif;
    background: var(--primary-gradient);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
    overflow-x: hidden;
    transition: background 0.5s ease;
}

.background-animation {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    overflow: hidden;
}

.gradient-orb {
    position: absolute;
    border-radius: 50%;
    filter: blur(80px);
    opacity: 0.5;
    animation: float 20s infinite ease-in-out;
}

.orb-1 {
    width: 400px;
    height: 400px;
    background: var(--bg-primary);
    top: -100px;
    left: -100px;
    animation-delay: 0s;
}

.orb-2 {
    width: 300px;
    height: 300px;
    background: var(--bg-secondary);
    bottom: -50px;
    right: -50px;
    animation-delay: -7s;
}

.orb-3 {
    width: 250px;
    height: 250px;
    background: #f093fb;
    top: 50%;
    left: 50%;
    animation-delay: -14s;
}

@keyframes float {
    0%, 100% { transform: translate(0, 0) scale(1); }
    25% { transform: translate(50px, -50px) scale(1.1); }
    50% { transform: translate(-30px, 30px) scale(0.9); }
    75% { transform: translate(-50px, -30px) scale(1.05); }
}

/* ========================================
   Container
======================================== */
.container {
    display: flex;
    flex-direction: column;
    align-items: center;
    max-width: 1200px;
    width: 100%;
    position: relative;
    z-index: 1;
}

/* ========================================
   Main Navigation
======================================== */
.main-nav {
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    padding: 15px;
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(20px);
    border-radius: 50px;
    box-shadow: var(--shadow-md);
    margin-bottom: 20px;
    justify-content: center;
}

.nav-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 6px;
    padding: 12px 20px;
    border: none;
    border-radius: 25px;
    background: rgba(255, 255, 255, 0.3);
    color: var(--text-light);
    font-size: 0.85rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.3s ease;
    min-width: 90px;
}

.nav-btn i {
    font-size: 1.3rem;
}

.nav-btn:hover {
    transform: translateY(-3px);
    box-shadow: var(--shadow-md);
    background: rgba(255, 255, 255, 0.5);
}

.nav-btn.active {
    background: var(--calculator-bg);
    color: var(--bg-primary);
    box-shadow: var(--shadow-lg);
    transform: translateY(-3px);
}

/* ========================================
   Tool Sections
======================================== */
.tool-section {
    display: none;
    width: 100%;
    animation: fadeIn 0.4s ease;
}

.tool-section.active {
    display: flex;
    justify-content: center;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* ========================================
   Tool Card
======================================== */
.tool-card {
    background: var(--calculator-bg);
    border-radius: var(--border-radius-lg);
    padding: 30px;
    box-shadow: var(--shadow-xl);
    backdrop-filter: blur(20px);
    width: 100%;
    max-width: 500px;
    transition: all 0.3s ease;
}

.tool-header {
    display: flex;
    align-items: center;
    gap: 15px;
    margin-bottom: 25px;
    padding-bottom: 15px;
    border-bottom: 2px solid var(--history-bg);
}

.tool-header i {
    font-size: 2rem;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.tool-header h2 {
    font-size: 1.6rem;
    font-weight: 700;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.tool-content {
    display: flex;
    flex-direction: column;
    gap: 20px;
}

/* ========================================
   Input Groups
======================================== */
.input-group {
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.input-group label {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--text-primary);
}

.input-group label i {
    color: var(--bg-primary);
}

.input-group input,
.input-group select {
    padding: 14px 16px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-md);
    font-size: 1rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
    transition: all 0.2s ease;
}

.input-group input:focus,
.input-group select:focus {
    outline: none;
    border-color: var(--bg-primary);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

.input-row {
    display: flex;
    align-items: center;
    gap: 10px;
}

.input-row input {
    flex: 1;
    padding: 14px 16px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-md);
    font-size: 1rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
    transition: all 0.2s ease;
}

.input-row input:focus {
    outline: none;
    border-color: var(--bg-primary);
}

.input-separator {
    color: var(--text-secondary);
    font-weight: 600;
    white-space: nowrap;
}

.input-row-split {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 15px;
}

.input-row-inline {
    display: flex;
    gap: 10px;
}

.input-row-inline .input-group {
    flex: 1;
}

/* ========================================
   Tool Buttons
======================================== */
.tool-btn {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 10px;
    padding: 16px 24px;
    border: none;
    border-radius: var(--border-radius-md);
    background: var(--primary-gradient);
    color: var(--text-light);
    font-size: 1rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
    box-shadow: var(--shadow-md);
}

.tool-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

.tool-btn:active {
    transform: scale(0.98);
}

/* ========================================
   Result Box
======================================== */
.result-box {
    background: var(--history-bg);
    border-radius: var(--border-radius-md);
    padding: 20px;
    min-height: 80px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.result-box.show {
    flex-direction: column;
    gap: 10px;
}

.empty-result {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 10px;
    color: var(--text-secondary);
}

.empty-result i {
    font-size: 2rem;
    opacity: 0.5;
}

.result-content {
    width: 100%;
    text-align: center;
}

.result-value {
    font-size: 2rem;
    font-weight: 700;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    margin-bottom: 8px;
}

.result-label {
    font-size: 0.9rem;
    color: var(--text-secondary);
}

.result-details {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 10px;
    margin-top: 15px;
    width: 100%;
}

.result-detail-item {
    background: rgba(255, 255, 255, 0.5);
    padding: 12px;
    border-radius: 8px;
    text-align: center;
}

.result-detail-item .label {
    font-size: 0.75rem;
    color: var(--text-secondary);
    margin-bottom: 4px;
}

.result-detail-item .value {
    font-size: 1.1rem;
    font-weight: 600;
    color: var(--text-primary);
}

/* ========================================
   Calc Tabs
======================================== */
.calc-tabs {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
}

.calc-tab {
    flex: 1;
    min-width: 100px;
    padding: 12px 16px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    background: transparent;
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
}

.calc-tab:hover {
    border-color: var(--bg-primary);
    color: var(--bg-primary);
}

.calc-tab.active {
    background: var(--primary-gradient);
    border-color: transparent;
    color: var(--text-light);
}

.calc-form {
    display: none;
    flex-direction: column;
    gap: 15px;
}

.calc-form.active {
    display: flex;
}

/* ========================================
   Unit Toggle
======================================== */
.unit-toggle {
    display: flex;
    gap: 10px;
}

.unit-btn {
    flex: 1;
    padding: 12px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    background: transparent;
    color: var(--text-secondary);
    font-size: 0.9rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
}

.unit-btn:hover {
    border-color: var(--bg-primary);
    color: var(--bg-primary);
}

.unit-btn.active {
    background: var(--primary-gradient);
    border-color: transparent;
    color: var(--text-light);
}

.hidden {
    display: none !important;
}

/* ========================================
   BMI Chart
======================================== */
.bmi-chart {
    background: var(--history-bg);
    border-radius: var(--border-radius-md);
    padding: 15px;
}

.bmi-chart h4 {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.9rem;
    color: var(--text-primary);
    margin-bottom: 12px;
}

.bmi-chart h4 i {
    color: var(--bg-primary);
}

.chart-row {
    display: flex;
    justify-content: space-between;
    padding: 10px 14px;
    border-radius: 6px;
    margin-bottom: 6px;
    font-size: 0.85rem;
    font-weight: 500;
}

.chart-row:last-child {
    margin-bottom: 0;
}

.chart-row.underweight {
    background: rgba(79, 172, 254, 0.2);
    color: #0c85d0;
}

.chart-row.normal {
    background: rgba(56, 239, 125, 0.2);
    color: #0d8a4a;
}

.chart-row.overweight {
    background: rgba(255, 154, 158, 0.2);
    color: #c0392b;
}

.chart-row.obese {
    background: rgba(231, 76, 60, 0.2);
    color: #922b21;
}

/* ========================================
   Tip Options
======================================== */
.tip-options {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
    align-items: center;
}

.tip-btn {
    padding: 10px 18px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    background: transparent;
    color: var(--text-secondary);
    font-size: 0.9rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
}

.tip-btn:hover {
    border-color: var(--bg-primary);
    color: var(--bg-primary);
}

.tip-btn.active {
    background: var(--primary-gradient);
    border-color: transparent;
    color: var(--text-light);
}

.tip-options input {
    flex: 1;
    min-width: 100px;
    padding: 10px 14px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    font-size: 0.9rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
}

.tip-options input:focus {
    outline: none;
    border-color: var(--bg-primary);
}

/* ========================================
   Converter Tabs
======================================== */
.converter-tabs {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
    margin-bottom: 20px;
}

.converter-tab {
    flex: 1;
    min-width: 80px;
    padding: 10px 14px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    background: transparent;
    color: var(--text-secondary);
    font-size: 0.8rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 4px;
}

.converter-tab i {
    font-size: 1rem;
}

.converter-tab:hover {
    border-color: var(--bg-primary);
    color: var(--bg-primary);
}

.converter-tab.active {
    background: var(--primary-gradient);
    border-color: transparent;
    color: var(--text-light);
}

.converter-form {
    display: flex;
    flex-direction: column;
    gap: 15px;
}

.convert-row {
    display: flex;
    gap: 10px;
}

.convert-row input {
    flex: 1;
}

.convert-row select {
    min-width: 120px;
    padding: 14px 12px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-md);
    font-size: 0.9rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
    cursor: pointer;
}

.convert-row select:focus {
    outline: none;
    border-color: var(--bg-primary);
}

.conversion-arrow {
    display: flex;
    justify-content: center;
    align-items: center;
    color: var(--bg-primary);
    font-size: 1.5rem;
}

/* ========================================
   Theme Switcher
======================================== */
.theme-switcher {
    position: fixed;
    top: 20px;
    left: 50%;
    transform: translateX(-50%);
    display: flex;
    gap: 10px;
    padding: 10px 15px;
    background: rgba(255, 255, 255, 0.2);
    backdrop-filter: blur(20px);
    border-radius: 50px;
    box-shadow: var(--shadow-md);
    z-index: 100;
}

.theme-btn {
    width: 32px;
    height: 32px;
    border: 2px solid transparent;
    border-radius: 50%;
    background: transparent;
    cursor: pointer;
    padding: 0;
    transition: all 0.3s ease;
    display: flex;
    align-items: center;
    justify-content: center;
}

.theme-btn.active {
    border-color: #ffffff;
    transform: scale(1.15);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
}

.theme-preview {
    width: 24px;
    height: 24px;
    border-radius: 50%;
}

.theme-preview.purple { background: linear-gradient(135deg, #667eea, #764ba2); }
.theme-preview.ocean { background: linear-gradient(135deg, #2193b0, #6dd5ed); }
.theme-preview.sunset { background: linear-gradient(135deg, #f093fb, #f6d365); }
.theme-preview.forest { background: linear-gradient(135deg, #11998e, #38ef7d); }
.theme-preview.dark { background: linear-gradient(135deg, #232526, #414345); }

/* ========================================
   Calculator
======================================== */
.calculator {
    background: var(--calculator-bg);
    border-radius: var(--border-radius-lg);
    padding: 24px;
    box-shadow: var(--shadow-xl);
    backdrop-filter: blur(20px);
    width: 100%;
    max-width: 500px;
    animation: slideUp 0.6s cubic-bezier(0.34, 1.56, 0.64, 1);
    transition: all 0.3s ease;
}

@keyframes slideUp {
    from {
        opacity: 0;
        transform: translateY(50px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* Calculator Header */
.calculator-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
}

.logo {
    display: flex;
    align-items: center;
    gap: 10px;
    font-size: 1.4rem;
    font-weight: 700;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.logo i {
    font-size: 1.6rem;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.header-actions {
    display: flex;
    gap: 8px;
}

.header-btn {
    width: 42px;
    height: 42px;
    border: none;
    border-radius: var(--border-radius-md);
    background: var(--btn-func-bg);
    color: var(--text-primary);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1rem;
    transition: all 0.2s ease;
    position: relative;
}

.header-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    background: var(--primary-gradient);
    color: var(--text-light);
}

.header-btn:active {
    transform: scale(0.95);
}

.header-btn i {
    font-size: 1.1rem;
}

.header-btn .mode-label {
    font-size: 0.7rem;
    position: absolute;
    bottom: 4px;
}

/* ========================================
   Display Section
======================================== */
.display-section {
    background: var(--display-bg);
    border-radius: var(--border-radius-lg);
    padding: 20px;
    margin-bottom: 16px;
    box-shadow: inset 0 4px 12px rgba(0, 0, 0, 0.2);
    position: relative;
    overflow: hidden;
}

.display-section::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 2px;
    background: linear-gradient(90deg, transparent, rgba(255,255,255,0.3), transparent);
}

.display-top {
    display: flex;
    justify-content: flex-start;
    align-items: center;
    margin-bottom: 10px;
}

.memory-indicator {
    display: flex;
    align-items: center;
    gap: 6px;
    font-size: 0.75rem;
    color: rgba(255, 255, 255, 0.6);
    padding: 4px 10px;
    background: rgba(255, 255, 255, 0.1);
    border-radius: 20px;
}

.memory-indicator.active {
    color: #00ff88;
    background: rgba(0, 255, 136, 0.2);
}

.expression-display {
    min-height: 24px;
    font-size: 0.9rem;
    color: rgba(255, 255, 255, 0.6);
    text-align: right;
    word-break: break-all;
    margin-bottom: 8px;
}

.main-display-container {
    min-height: 60px;
    display: flex;
    align-items: center;
    justify-content: flex-end;
}

.main-display {
    font-size: 3rem;
    font-weight: 700;
    color: var(--text-light);
    word-break: break-all;
    text-align: right;
    transition: all 0.2s ease;
    line-height: 1.2;
}

.main-display.animate {
    animation: pulse 0.3s ease;
}

@keyframes pulse {
    0%, 100% { transform: scale(1); opacity: 1; }
    50% { transform: scale(1.02); opacity: 0.9; }
}

.display-hints {
    min-height: 20px;
    font-size: 0.8rem;
    color: rgba(255, 255, 255, 0.5);
    text-align: right;
    margin-top: 8px;
}

/* ========================================
   Memory Row
======================================== */
.memory-row {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 8px;
    margin-bottom: 12px;
}

.btn-memory {
    padding: 10px;
    border: none;
    border-radius: var(--border-radius-sm);
    background: var(--btn-func-bg);
    color: var(--text-primary);
    font-size: 0.75rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.2s ease;
}

.btn-memory:hover {
    background: #e0e0e0;
    transform: translateY(-2px);
    box-shadow: var(--shadow-sm);
}

.btn-memory:active {
    transform: scale(0.95);
}

/* ========================================
   Scientific Rows
======================================== */
.scientific-row {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 8px;
    margin-bottom: 12px;
}

.scientific-row.hidden {
    display: none;
}

/* ========================================
   Buttons Grid
======================================== */
.buttons-grid {
    display: grid;
    grid-template-columns: repeat(5, 1fr);
    gap: 10px;
}

.btn {
    padding: 18px;
    border: none;
    border-radius: var(--border-radius-md);
    font-size: 1.2rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    cursor: pointer;
    transition: all 0.15s ease;
    position: relative;
    overflow: hidden;
}

.btn::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(180deg, rgba(255,255,255,0.2) 0%, transparent 100%);
    border-radius: var(--border-radius-md);
    pointer-events: none;
}

.btn:active {
    transform: scale(0.92);
}

.btn-num {
    background: var(--btn-num-bg);
    color: var(--btn-num-color);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.btn-num:hover {
    background: #f8f9fa;
    box-shadow: 0 6px 16px rgba(0, 0, 0, 0.12);
    transform: translateY(-2px);
}

.btn-op {
    background: var(--btn-op-bg);
    color: var(--btn-op-color);
    box-shadow: 0 4px 12px rgba(102, 126, 234, 0.3);
}

.btn-op:hover {
    box-shadow: 0 6px 16px rgba(102, 126, 234, 0.5);
    transform: translateY(-2px);
}

.btn-func {
    background: var(--btn-func-bg);
    color: var(--btn-func-color);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.btn-func:hover {
    background: #e0e0e0;
    box-shadow: 0 6px 16px rgba(0, 0, 0, 0.12);
    transform: translateY(-2px);
}

.btn-equals {
    background: var(--btn-equals-bg);
    color: var(--text-light);
    box-shadow: 0 4px 12px rgba(240, 147, 251, 0.4);
}

.btn-equals:hover {
    box-shadow: 0 6px 16px rgba(240, 147, 251, 0.6);
    transform: translateY(-2px);
}

.btn-sci {
    background: var(--btn-sci-bg);
    color: var(--btn-sci-color);
    font-size: 0.9rem;
    padding: 14px 10px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.btn-sci:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 16px rgba(0, 0, 0, 0.12);
}

.btn-sci-small {
    background: var(--btn-func-bg);
    color: var(--text-primary);
    font-size: 1rem;
    padding: 14px 10px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.btn-sci-small:hover {
    background: #e0e0e0;
    transform: translateY(-2px);
}

/* ========================================
   Panel Overlay (Popup Background)
======================================== */
.panel-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: var(--overlay-bg);
    backdrop-filter: blur(8px);
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 1000;
    opacity: 0;
    visibility: hidden;
    transition: all 0.3s ease;
}

.panel-overlay.active {
    opacity: 1;
    visibility: visible;
}

/* ========================================
   Panel (Popup)
======================================== */
.panel {
    background: var(--panel-bg);
    border-radius: var(--border-radius-lg);
    box-shadow: var(--shadow-xl);
    backdrop-filter: blur(20px);
    width: 90%;
    max-width: 450px;
    max-height: 80vh;
    display: flex;
    flex-direction: column;
    transform: scale(0.9) translateY(-20px);
    transition: all 0.3s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.panel-overlay.active .panel {
    transform: scale(1) translateY(0);
}

.panel-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 20px 24px;
    border-bottom: 2px solid var(--history-bg);
}

.panel-title {
    display: flex;
    align-items: center;
    gap: 12px;
    font-size: 1.3rem;
    font-weight: 600;
    color: var(--text-primary);
}

.panel-title i {
    color: var(--bg-primary);
    font-size: 1.4rem;
}

.panel-actions {
    display: flex;
    gap: 8px;
}

.panel-close,
.panel-btn {
    width: 38px;
    height: 38px;
    border: none;
    border-radius: var(--border-radius-sm);
    background: var(--btn-func-bg);
    color: var(--text-primary);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1rem;
    transition: all 0.2s ease;
}

.panel-close:hover,
.panel-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.panel-btn.danger:hover {
    background: #ffe0e0;
    color: #e74c3c;
}

.panel-btn:hover {
    background: var(--primary-gradient);
    color: var(--text-light);
}

.panel-content {
    padding: 24px;
    overflow-y: auto;
    max-height: calc(80vh - 80px);
}

.panel-content::-webkit-scrollbar {
    width: 6px;
}

.panel-content::-webkit-scrollbar-track {
    background: var(--history-bg);
    border-radius: 10px;
}

.panel-content::-webkit-scrollbar-thumb {
    background: var(--bg-primary);
    border-radius: 10px;
}

/* Keyboard Hints */
.keyboard-hints {
    margin-bottom: 20px;
    padding: 16px;
    background: var(--history-bg);
    border-radius: var(--border-radius-md);
}

.keyboard-hints h4 {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.95rem;
    color: var(--text-primary);
    margin-bottom: 12px;
}

.keyboard-hints h4 i {
    color: var(--bg-primary);
}

.hints-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 10px;
}

.hint-item {
    display: flex;
    align-items: center;
    gap: 6px;
    font-size: 0.85rem;
    color: var(--text-secondary);
}

.hint-item kbd {
    display: inline-block;
    padding: 4px 8px;
    background: var(--btn-func-bg);
    border-radius: 4px;
    font-family: 'Inter', sans-serif;
    font-size: 0.75rem;
    font-weight: 600;
    color: var(--text-primary);
    border: 1px solid rgba(0, 0, 0, 0.1);
}

/* ========================================
   Settings Panel Specific
======================================== */
.setting-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 16px 0;
    border-bottom: 1px solid var(--history-bg);
}

.setting-item:last-child {
    border-bottom: none;
}

.setting-label {
    display: flex;
    align-items: center;
    gap: 12px;
    font-size: 0.95rem;
    color: var(--text-primary);
}

.setting-label i {
    color: var(--bg-primary);
    font-size: 1.1rem;
}

.toggle-switch {
    position: relative;
    width: 50px;
    height: 26px;
}

.toggle-switch input {
    opacity: 0;
    width: 0;
    height: 0;
}

.toggle-slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #ccc;
    transition: 0.3s;
    border-radius: 26px;
}

.toggle-slider:before {
    position: absolute;
    content: "";
    height: 20px;
    width: 20px;
    left: 3px;
    bottom: 3px;
    background-color: white;
    transition: 0.3s;
    border-radius: 50%;
}

.toggle-switch input:checked + .toggle-slider {
    background: var(--primary-gradient);
}

.toggle-switch input:checked + .toggle-slider:before {
    transform: translateX(24px);
}

.setting-select {
    padding: 8px 14px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    font-size: 0.9rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
}

.setting-select:focus {
    outline: none;
    border-color: var(--bg-primary);
}

/* ========================================
   History Panel Specific
======================================== */
.history-stats {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
    margin-bottom: 16px;
}

.stat-item {
    background: var(--history-bg);
    border-radius: var(--border-radius-md);
    padding: 16px;
    text-align: center;
}

.stat-value {
    display: block;
    font-size: 1.6rem;
    font-weight: 700;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.stat-label {
    font-size: 0.8rem;
    color: var(--text-secondary);
}

.search-box {
    position: relative;
    margin-bottom: 12px;
}

.search-box i {
    position: absolute;
    left: 14px;
    top: 50%;
    transform: translateY(-50%);
    color: var(--text-secondary);
}

.search-box input {
    width: 100%;
    padding: 12px 12px 12px 42px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-md);
    font-size: 0.9rem;
    font-family: 'Inter', sans-serif;
    background: var(--history-bg);
    color: var(--text-primary);
    transition: all 0.2s ease;
}

.search-box input:focus {
    outline: none;
    border-color: var(--bg-primary);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
}

.history-filters {
    display: flex;
    gap: 8px;
    margin-bottom: 16px;
}

.filter-btn {
    flex: 1;
    padding: 10px;
    border: 2px solid var(--history-bg);
    border-radius: var(--border-radius-sm);
    background: transparent;
    color: var(--text-secondary);
    font-size: 0.85rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
}

.filter-btn:hover {
    border-color: var(--bg-primary);
    color: var(--bg-primary);
}

.filter-btn.active {
    background: var(--primary-gradient);
    border-color: transparent;
    color: var(--text-light);
}

.history-list {
    max-height: 350px;
    overflow-y: auto;
    padding-right: 4px;
}

.history-list::-webkit-scrollbar {
    width: 4px;
}

.history-list::-webkit-scrollbar-thumb {
    background: var(--bg-primary);
    border-radius: 4px;
}

.history-item {
    background: var(--history-bg);
    border-radius: var(--border-radius-md);
    padding: 14px;
    margin-bottom: 10px;
    cursor: pointer;
    transition: all 0.2s ease;
    animation: slideIn 0.3s ease;
    border-left: 4px solid transparent;
}

@keyframes slideIn {
    from {
        opacity: 0;
        transform: translateX(-20px);
    }
    to {
        opacity: 1;
        transform: translateX(0);
    }
}

.history-item:hover {
    background: var(--history-item-hover);
    transform: translateX(8px);
    box-shadow: var(--shadow-sm);
    border-left-color: var(--bg-primary);
}

.history-expression {
    font-size: 0.85rem;
    color: var(--text-secondary);
    margin-bottom: 6px;
    word-break: break-all;
}

.history-result {
    font-size: 1.2rem;
    font-weight: 600;
    color: var(--text-primary);
    text-align: right;
}

.history-time {
    font-size: 0.75rem;
    color: var(--text-secondary);
    margin-top: 6px;
    text-align: right;
}

.empty-state {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 50px 20px;
    color: var(--text-secondary);
    text-align: center;
}

.empty-icon {
    width: 80px;
    height: 80px;
    background: var(--history-bg);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-bottom: 20px;
}

.empty-icon i {
    font-size: 2.5rem;
    color: var(--bg-primary);
}

.empty-state h3 {
    font-size: 1.2rem;
    margin-bottom: 8px;
}

.empty-state p {
    font-size: 0.9rem;
}

/* ========================================
   Toast Notification
======================================== */
.toast {
    position: fixed;
    bottom: 30px;
    left: 50%;
    transform: translateX(-50%) translateY(100px);
    background: var(--calculator-bg);
    color: var(--text-primary);
    padding: 14px 28px;
    border-radius: 50px;
    box-shadow: var(--shadow-xl);
    display: flex;
    align-items: center;
    gap: 12px;
    font-size: 0.95rem;
    font-weight: 500;
    z-index: 2000;
    opacity: 0;
    transition: all 0.3s cubic-bezier(0.68, -0.55, 0.265, 1.55);
    min-width: 200px;
}

.toast.show {
    transform: translateX(-50%) translateY(0);
    opacity: 1;
}

.toast i {
    color: #00b894;
    font-size: 1.2rem;
}

/* ========================================
   Responsive Design
======================================== */
@media (max-width: 768px) {
    .main-nav {
        gap: 6px;
        padding: 10px;
    }

    .nav-btn {
        min-width: 70px;
        padding: 10px 14px;
        font-size: 0.75rem;
    }

    .nav-btn i {
        font-size: 1.1rem;
    }

    .calculator {
        padding: 16px;
    }

    .btn {
        padding: 14px;
        font-size: 1rem;
    }

    .main-display {
        font-size: 2rem;
    }

    .tool-card {
        padding: 20px;
    }

    .input-row-split {
        grid-template-columns: 1fr;
    }

    .converter-tab {
        min-width: 60px;
        font-size: 0.7rem;
    }
}

@media (max-width: 480px) {
    .scientific-row {
        grid-template-columns: repeat(3, 1fr);
    }

    .buttons-grid {
        grid-template-columns: repeat(4, 1fr);
    }

    .memory-row {
        grid-template-columns: repeat(5, 1fr);
    }

    .btn-sci {
        font-size: 0.8rem;
        padding: 10px 6px;
    }
}