:root {
            --bg-primary: #0f0c29;
            --bg-secondary: #302b63;
            --bg-tertiary: #24243e;
            --card-bg: rgba(255,255,255,0.97);
            --card-border: rgba(255,255,255,0.15);
            --text-primary: #1e293b;
            --text-secondary: #475569;
            --text-heading: #1d4ed8;
            --accent: #6366f1;
            --accent-light: #818cf8;
            --energy-orange: #f97316;
            --energy-yellow: #facc15;
            --success: #10b981;
            --error: #ef4444;
            --warning: #f59e0b;
            --radius: 16px;
            --shadow: 0 8px 32px rgba(0,0,0,0.12);
            --shadow-lg: 0 20px 50px rgba(0,0,0,0.2);
            --transition: 0.3s cubic-bezier(.4,0,.2,1);
        }
        .dark-mode {
            --card-bg: rgba(30,32,48,0.97);
            --card-border: rgba(255,255,255,0.08);
            --text-primary: #e2e8f0;
            --text-secondary: #94a3b8;
            --text-heading: #818cf8;
            --shadow: 0 8px 32px rgba(0,0,0,0.4);
        }
        * { box-sizing: border-box; margin: 0; padding: 0; }
        body {
            font-family: 'Inter', system-ui, sans-serif;
            line-height: 1.7;
            color: var(--text-primary);
            background: linear-gradient(135deg, var(--bg-primary), var(--bg-secondary), var(--bg-tertiary));
            min-height: 100vh;
        }
        /* Progress Bar */
        #progressBar {
            position: fixed; top: 0; left: 0; height: 3px; z-index: 1001;
            background: linear-gradient(90deg, #6366f1, #06b6d4, #10b981);
            transition: width 0.1s linear; width: 0%;
        }
        /* Navigation */
        nav {
            position: sticky; top: 64px; z-index: 10;
            background: rgba(15,12,41,0.92); backdrop-filter: blur(16px);
            border-bottom: 1px solid rgba(255,255,255,0.08);
            padding: 0 24px;
        }
        .nav-inner {
            max-width: 960px; margin: 0 auto;
            display: flex; align-items: center; justify-content: space-between;
            height: 56px; gap: 12px;
        }
        .nav-brand { color: #fff; font-weight: 700; font-size: 1.1rem; white-space: nowrap; }
        .nav-links { display: flex; gap: 4px; overflow-x: auto; scrollbar-width: none; }
        .nav-links::-webkit-scrollbar { display: none; }
        .nav-links a {
            color: rgba(255,255,255,0.7); text-decoration: none; font-size: 0.82rem;
            padding: 6px 12px; border-radius: 8px; white-space: nowrap;
            transition: var(--transition); font-weight: 500;
        }
        .nav-links a:hover, .nav-links a.active { color: #fff; background: rgba(99,102,241,0.3); }
        .nav-actions { display: flex; gap: 8px; align-items: center; }
        .theme-toggle {
            background: rgba(255,255,255,0.1); border: none; color: #fff;
            width: 36px; height: 36px; border-radius: 50%; cursor: pointer;
            font-size: 1.1rem; display: flex; align-items: center; justify-content: center;
            transition: var(--transition);
        }
        .theme-toggle:hover { background: rgba(255,255,255,0.2); transform: rotate(20deg); }
        /* Floating BG */
        .floating-bg { position: fixed; inset: 0; pointer-events: none; z-index: -1; overflow: hidden; }
        .orb {
            position: absolute; border-radius: 50%;
            background: radial-gradient(circle, rgba(99,102,241,0.15), transparent 70%);
            animation: orbFloat 12s ease-in-out infinite;
        }
        .orb:nth-child(1) { width: 300px; height: 300px; top: 10%; left: 5%; animation-delay: 0s; }
        .orb:nth-child(2) { width: 200px; height: 200px; top: 60%; right: 10%; animation-delay: 4s;
            background: radial-gradient(circle, rgba(6,182,212,0.12), transparent 70%); }
        .orb:nth-child(3) { width: 250px; height: 250px; top: 40%; left: 50%; animation-delay: 8s;
            background: radial-gradient(circle, rgba(249,115,22,0.1), transparent 70%); }
        @keyframes orbFloat {
            0%, 100% { transform: translateY(0) scale(1); }
            50% { transform: translateY(-40px) scale(1.1); }
        }
        /* Container */
        .container { max-width: 920px; margin: 0 auto; padding: 24px 20px 60px; position: relative; z-index: 1; }
        /* Section titles */
        .section-title {
            text-align: center; padding: 48px 0 12px;
            font-size: clamp(2rem, 4vw, 3rem); font-weight: 800;
            background: linear-gradient(135deg, #fff, #cbd5e1);
            -webkit-background-clip: text; -webkit-text-fill-color: transparent;
            background-clip: text;
        }
        .section-subtitle { text-align: center; color: rgba(255,255,255,0.6); margin-bottom: 32px; font-size: 1.05rem; }
        /* Cards */
        .card {
            background: var(--card-bg); border-radius: var(--radius);
            box-shadow: var(--shadow); padding: 36px; margin: 24px 0;
            border: 1px solid var(--card-border);
            transition: transform var(--transition), box-shadow var(--transition);
            position: relative; overflow: hidden;
        }
        .card::before {
            content: ''; position: absolute; top: 0; left: 0; right: 0; height: 3px;
            background: linear-gradient(90deg, var(--accent), #06b6d4);
        }
        .card:hover { transform: translateY(-4px); box-shadow: var(--shadow-lg); }
        .card h2 {
            font-size: 1.6rem; font-weight: 700; color: var(--text-heading);
            margin-bottom: 16px; display: flex; align-items: center; gap: 10px;
        }
        .card h3 {
            font-size: 1.25rem; font-weight: 600; color: var(--accent);
            margin: 28px 0 14px; padding-left: 16px;
            border-left: 3px solid var(--accent-light);
        }
        .card p { margin-bottom: 12px; color: var(--text-secondary); }
        .card ul { padding-left: 24px; margin-bottom: 12px; color: var(--text-secondary); }
        .card li { margin-bottom: 6px; }
        /* Boxes */
        .info-box {
            padding: 16px 20px; border-radius: 12px; margin: 16px 0;
            border-left: 4px solid; position: relative;
        }
        .info-box.learning { background: linear-gradient(135deg,#eef2ff,#e0e7ff); border-color: var(--accent); }
        .info-box.example  { background: linear-gradient(135deg,#fefce8,#fef3c7); border-color: var(--warning); }
        .info-box.formula  { background: linear-gradient(135deg,#f0fdf4,#dcfce7); border-color: var(--success); }
        .dark-mode .info-box.learning { background: rgba(99,102,241,0.1); }
        .dark-mode .info-box.example  { background: rgba(245,158,11,0.1); }
        .dark-mode .info-box.formula  { background: rgba(16,185,129,0.1); }
        .info-box h4 { font-size: 0.95rem; font-weight: 600; margin-bottom: 6px; }
        .info-box li { font-size: 0.95rem; }
        /* Simulation containers */
        .sim-wrapper {
            margin: 20px 0; border-radius: 12px; overflow: hidden;
            background: #0d1117; border: 1px solid rgba(255,255,255,0.06);
        }
        .sim-controls {
            display: flex; flex-wrap: wrap; align-items: center; gap: 10px;
            padding: 12px 16px; background: rgba(255,255,255,0.03);
            border-bottom: 1px solid rgba(255,255,255,0.06);
        }
        .sim-canvas {
            width: 100%; display: block; cursor: default;
            image-rendering: auto;
        }
        .sim-info {
            padding: 10px 16px; font-size: 0.85rem; color: rgba(255,255,255,0.5);
            background: rgba(255,255,255,0.02); border-top: 1px solid rgba(255,255,255,0.06);
        }
        .sim-stat {
            display: inline-flex; align-items: center; gap: 6px;
            background: rgba(255,255,255,0.06); padding: 4px 12px; border-radius: 20px;
            font-size: 0.82rem; color: rgba(255,255,255,0.8); font-weight: 500;
        }
        /* Buttons */
        .btn {
            display: inline-flex; align-items: center; gap: 6px;
            padding: 8px 20px; border-radius: 10px; border: none;
            font-family: inherit; font-weight: 600; font-size: 0.9rem;
            cursor: pointer; transition: var(--transition);
        }
        .btn-primary { background: linear-gradient(135deg,#6366f1,#8b5cf6); color: #fff; }
        .btn-primary:hover { transform: translateY(-2px); box-shadow: 0 8px 20px rgba(99,102,241,0.4); }
        .btn-success { background: linear-gradient(135deg,#10b981,#059669); color: #fff; }
        .btn-warning { background: linear-gradient(135deg,#f59e0b,#d97706); color: #fff; }
        .btn-danger  { background: linear-gradient(135deg,#ef4444,#dc2626); color: #fff; }
        .btn-outline {
            background: transparent; color: rgba(255,255,255,0.8);
            border: 1px solid rgba(255,255,255,0.2);
        }
        .btn-outline:hover { background: rgba(255,255,255,0.1); }
        .btn:disabled { opacity: 0.5; cursor: not-allowed; transform: none !important; }
        .play-btn {
            display: block; margin: 20px auto; padding: 12px 32px;
            font-size: 1rem; border-radius: 12px;
        }
        /* Slider */
        .sim-slider {
            -webkit-appearance: none; appearance: none;
            height: 6px; border-radius: 3px; background: rgba(255,255,255,0.15);
            outline: none; flex: 1; min-width: 120px;
        }
        .sim-slider::-webkit-slider-thumb {
            -webkit-appearance: none; width: 18px; height: 18px;
            border-radius: 50%; background: var(--accent-light); cursor: pointer;
            border: 2px solid #fff;
        }
        .sim-label { color: rgba(255,255,255,0.7); font-size: 0.82rem; font-weight: 500; white-space: nowrap; }
        /* Calculator */
        /* Calculator result */
        .calc-result {
            background: linear-gradient(135deg,#eef2ff,#e0e7ff);
            padding: 20px; border-radius: 12px; margin-top: 16px;
        }
        .dark-mode .calc-result { background: rgba(99,102,241,0.1); }
        .calc-result .big-number { font-size: 1.6rem; font-weight: 700; color: var(--accent); }
        /* Comparison table */
        .cmp-table { width: 100%; border-collapse: collapse; margin: 20px 0; border-radius: 12px; overflow: hidden; }
        .cmp-table th, .cmp-table td { padding: 12px 16px; text-align: left; border-bottom: 1px solid rgba(99,102,241,0.1); }
        .cmp-table th { background: linear-gradient(135deg,#6366f1,#8b5cf6); color: #fff; font-weight: 600; font-size: 0.9rem; }
        .cmp-table td { font-size: 0.9rem; color: var(--text-secondary); }
        .cmp-table tr:hover td { background: rgba(99,102,241,0.04); }
        /* Quiz */
        .quiz-box {
            background: linear-gradient(135deg,#f0f9ff,#e0f2fe);
            border-radius: 14px; padding: 28px; border: 1px solid rgba(6,182,212,0.15);
        }
        .dark-mode .quiz-box { background: rgba(6,182,212,0.06); }
        .q-item {
            background: var(--card-bg); padding: 18px; border-radius: 12px;
            margin-bottom: 16px; border-left: 4px solid transparent;
            transition: var(--transition);
        }
        .q-item:hover { border-left-color: var(--accent); }
        .q-item p { font-weight: 600; margin-bottom: 10px; color: var(--text-primary); }
        .q-item label {
            display: flex; align-items: center; gap: 8px; padding: 6px 10px;
            border-radius: 8px; cursor: pointer; transition: var(--transition);
            font-size: 0.95rem; color: var(--text-secondary);
        }
        .q-item label:hover { background: rgba(99,102,241,0.06); }
        .q-item input[type="radio"] { accent-color: var(--accent); width: 18px; height: 18px; }
        .q-result {
            margin-top: 20px; padding: 20px; border-radius: 12px;
            background: var(--card-bg); display: none;
        }
        .q-result.show { display: block; animation: slideUp 0.4s ease-out; }
        .q-correct { color: var(--success); }
        .q-wrong { color: var(--error); }
        @keyframes slideUp { from { opacity: 0; transform: translateY(20px); } to { opacity: 1; transform: translateY(0); } }
        /* Scroll reveal */
        .reveal { opacity: 0; transform: translateY(30px); transition: all 0.7s cubic-bezier(.4,0,.2,1); }
        .reveal.visible { opacity: 1; transform: translateY(0); }
        /* Footer */
        footer {
            text-align: center; padding: 40px 20px; color: rgba(255,255,255,0.4);
            font-size: 0.85rem; border-top: 1px solid rgba(255,255,255,0.06);
        }
        /* Responsive */
        @media (max-width: 768px) {
            .card { padding: 24px 18px; }
            /* responsive spacing */
            .nav-links { display: none; }
            .section-title { font-size: 1.8rem; }
        }