* { margin:0; padding:0; box-sizing:border-box; }
        body {
            background: linear-gradient(135deg, #e8f4f8 0%, #d1e9f0 100%);
            font-family: system-ui, -apple-system, 'Segoe UI', sans-serif;
            padding: 20px;
        }
        .double-border {
            width: min(1400px, 100%);
            margin: 0 auto;
            background: #ffffff;
            border: 2px solid #f1c40f;
            border-radius: 24px;
            box-shadow: 0 20px 30px -15px rgba(0,0,0,0.2), inset 0 1px 0 0 #fff8e0;
            overflow: hidden;
        }
        .container {
            display: grid;
            grid-template-columns: 260px minmax(0, 1fr);
            gap: 0;
        }
        .sidebar {
            background: #fefcf5;
            border-right: 2px solid #fdebb3;
            padding: 24px 16px;
        }
        .sidebar h3 {
            font-size: 1.4rem;
            color: #b45f06;
            border-left: 5px solid #f39c12;
            padding-left: 15px;
            margin-bottom: 25px;
        }
        .chapter-list {
            list-style: none;
        }
        .chapter-list li {
            margin-bottom: 6px;
        }
        .chapter-list a {
            display: block;
            padding: 8px 12px;
            text-decoration: none;
            color: #2c3e50;
            border-radius: 12px;
            transition: all 0.2s;
            font-weight: 500;
        }
        .chapter-list a:hover {
            background: #fdebd0;
            transform: translateX(3px);
        }
        .chapter-list .active {
            background: #3b82f6;
            color: white;
            box-shadow: 0 2px 5px rgba(59,130,246,0.3);
        }
        .main-content {
            padding: 30px 35px;
            background: white;
            min-width: 0;
            overflow-wrap: anywhere;
        }
        .course-heading {
            margin-bottom: 24px;
            padding-bottom: 16px;
            border-bottom: 2px solid #fdebb3;
        }
        .course-heading .course-title {
            font-size: 1.6rem;
            color: #92400e;
            font-weight: 700;
            margin-bottom: 8px;
        }
        .course-heading .chapter-title {
            font-size: 1.15rem;
            color: #1e3a8a;
            font-weight: 600;
        }
        .footer-nav {
            margin-top: 45px;
            padding-top: 20px;
            border-top: 2px solid #f0e6d2;
            display: flex;
            justify-content: space-between;
        }
        .footer-nav a {
            padding: 8px 24px;
            background: #f1f5f9;
            text-decoration: none;
            color: #1e40af;
            border-radius: 40px;
            font-weight: 500;
            transition: 0.2s;
        }
        .footer-nav a:hover {
            background: #e2e8f0;
            transform: translateY(-2px);
        }
        footer {
            text-align: center;
            margin-top: 30px;
            padding: 15px;
            color: #64748b;
            border-top: 1px dashed #e2e8f0;
            font-size: 0.9rem;
        }
        h2 { color: #1e3a8a; border-left: 6px solid #f59e0b; padding-left: 15px; margin: 20px 0 15px; }
        h3 { color: #2c3e50; margin: 20px 0 10px; }
        pre {
            background: #2d2f36; color: #f8f8f2; padding: 15px; border-radius: 12px;
            overflow-x: auto; font-family: 'JetBrains Mono', monospace; margin: 15px 0;
        }
        table {
            width: 100%; border-collapse: collapse; margin: 15px 0; background: #fefce8;
        }
        th, td {
            border: 1px solid #fcd34d; padding: 10px; text-align: left;
        }
        th { background: #fffbeb; }
        .tip, .warning, .highlight {
            padding: 12px 18px; border-radius: 16px; margin: 15px 0;
        }
        .tip { background: #e0f2fe; border-left: 6px solid #0ea5e9; }
        .warning { background: #fee2e2; border-left: 6px solid #ef4444; }
        .highlight { background: #fef9c3; border: 1px solid #facc15; }
        svg {
            max-width: 100%;
            height: auto;
            display: block;
            margin: 18px auto;
        }
        img, video, canvas {
            max-width: 100%;
            height: auto;
        }
        @media (max-width: 768px) {
            body {
                padding: 10px;
            }
            .double-border {
                border-radius: 14px;
            }
            .container {
                display: block;
            }
            .sidebar {
                border-right: none;
                border-bottom: 2px solid #fdebb3;
                padding: 16px 12px;
            }
            .sidebar h3 {
                font-size: 1.1rem;
                margin-bottom: 12px;
            }
            .chapter-list {
                display: grid;
                grid-template-columns: repeat(auto-fit, minmax(130px, 1fr));
                gap: 6px;
            }
            .chapter-list li {
                margin-bottom: 0;
            }
            .chapter-list a {
                padding: 8px;
                font-size: 0.92rem;
            }
            .main-content {
                padding: 18px 14px;
            }
            .course-heading .course-title {
                font-size: 1.25rem;
            }
            .course-heading .chapter-title {
                font-size: 1rem;
            }
            .footer-nav {
                flex-direction: column;
                gap: 10px;
            }
            .footer-nav a {
                text-align: center;
            }
            table {
                display: block;
                overflow-x: auto;
            }
        }