:root {
            /* Light Theme */
            --bg-body-light: #f0f2f5;
            --bg-card-light: #ffffff;
            --text-primary-light: #2c3e50;
            --text-secondary-light: #7f8c8d;
            --border-light: #e0e6ed;

            /* Dark Theme */
            --bg-body-dark: #1a202c;
            --bg-card-dark: #2d3748;
            --text-primary-dark: #e2e8f0;
            --text-secondary-dark: #a0aec0;
            --border-dark: #4a5568;

            /* Accent Colors */
            --primary-accent: #667eea;
            --secondary-accent: #805ad5;
            --highlight-color: #38b2ac;
            --warning-color: #f6ad55;

            /* Current Theme Variables (Default to Dark) */
            --bg-body: var(--bg-body-dark);
            --bg-card: var(--bg-card-dark);
            --text-primary: var(--text-primary-dark);
            --text-secondary: var(--text-secondary-dark);
            --border-color: var(--border-dark);
            --header-bg: var(--primary-accent);
        }

        /* Light theme active */
        body.light-theme {
            --bg-body: var(--bg-body-light);
            --bg-card: var(--bg-card-light);
            --text-primary: var(--text-primary-light);
            --text-secondary: var(--text-secondary-light);
            --border-color: var(--border-light);
        }

        *, *::before, *::after {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
        }

        body {
            font-family: 'Roboto', sans-serif;
            background-color: var(--bg-body);
            color: var(--text-primary);
            line-height: 1.6;
            transition: background-color 0.4s ease, color 0.4s ease;
            -webkit-font-smoothing: antialiased;
            -moz-osx-font-smoothing: grayscale;
        }

        /* --- Header --- */
        header {
            background: linear-gradient(135deg, var(--primary-accent), var(--secondary-accent));
            color: #ffffff;
            padding: 3.5rem 1.5rem 2.5rem;
            text-align: center;
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.25);
            animation: fadeInHeader 0.9s ease-out;
            position: relative;
            overflow: hidden;
        }

        header::before {
            content: '';
            position: absolute;
            top: -50%;
            left: -50%;
            width: 200%;
            height: 200%;
            background: rgba(255, 255, 255, 0.05);
            transform: rotate(45deg);
            z-index: 0;
            animation: headerOverlay 20s linear infinite;
        }

        header h1 {
            font-family: 'Montserrat', sans-serif;
            font-size: clamp(2.5rem, 6vw, 3.8rem);
            font-weight: 700;
            letter-spacing: -1.5px;
            text-shadow: 0 3px 8px rgba(0, 0, 0, 0.3);
            position: relative;
            z-index: 1;
        }

        header p {
            font-size: clamp(1rem, 2.5vw, 1.3rem);
            color: rgba(255, 255, 255, 0.9);
            margin-top: 0.8rem;
            position: relative;
            z-index: 1;
        }

        /* --- Navigation & Controls --- */
        .nav-controls {
            display: flex;
            justify-content: center;
            align-items: center;
            flex-wrap: wrap;
            background-color: var(--bg-card);
            padding: 1.2rem 1rem;
            gap: 1.5rem;
            border-bottom: 1px solid var(--border-color);
            box-shadow: 0 2px 10px rgba(0, 0, 0, 0.1);
            position: sticky;
            top: 0;
            z-index: 1000;
        }

        .nav-buttons {
            display: flex;
            gap: 1.5rem;
        }

        .nav-buttons button {
            background-color: var(--primary-accent);
            border: none;
            padding: 0.9rem 2.2rem;
            font-family: 'Montserrat', sans-serif;
            font-size: 1rem;
            border-radius: 0.6rem;
            color: #ffffff;
            font-weight: 600;
            cursor: pointer;
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
            transition: transform 0.3s ease, background-color 0.3s ease, box-shadow 0.3s ease;
        }

        .nav-buttons button:hover {
            background-color: var(--highlight-color);
            transform: translateY(-3px) scale(1.02);
            box-shadow: 0 6px 20px rgba(0, 0, 0, 0.3);
        }

        .nav-buttons button.active {
            background-color: var(--highlight-color);
            box-shadow: inset 0 2px 5px rgba(0, 0, 0, 0.2), 0 2px 10px rgba(0, 0, 0, 0.3);
            transform: translateY(0);
        }

        /* Theme Toggle */
        .theme-toggle {
            background: var(--bg-body);
            border: 1px solid var(--border-color);
            border-radius: 50px;
            width: 60px;
            height: 30px;
            display: flex;
            align-items: center;
            padding: 3px;
            cursor: pointer;
            position: relative;
            transition: background 0.3s ease, border-color 0.3s ease;
        }

        .theme-toggle-handle {
            width: 24px;
            height: 24px;
            background-color: var(--primary-accent);
            border-radius: 50%;
            transition: transform 0.3s ease, background-color 0.3s ease;
            display: flex;
            justify-content: center;
            align-items: center;
            color: #fff;
            font-size: 0.8em;
        }

        .theme-toggle.light-active .theme-toggle-handle {
            transform: translateX(30px);
            background-color: var(--highlight-color);
        }

        /* --- Main Content & Sections --- */
        .container {
            max-width: 1200px;
            margin: 3rem auto;
            padding: 0 1.5rem;
            transition: opacity 0.6s ease;
        }

        .container:not(.active) {
            display: none;
            opacity: 0;
        }

        .card {
            background: var(--bg-card);
            border-radius: 1rem;
            box-shadow: 0 8px 25px rgba(0, 0, 0, 0.2);
            border: 1px solid var(--border-color);
            transition: transform 0.3s cubic-bezier(0.25, 0.8, 0.25, 1), box-shadow 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
        }

        .card:hover {
            transform: translateY(-10px);
            box-shadow: 0 15px 40px rgba(0, 0, 0, 0.35);
        }

        .year {
            margin-bottom: 3.5rem;
            overflow: hidden;
            border-radius: 1rem;
        }

        .year h2 {
            font-family: 'Montserrat', sans-serif;
            font-size: clamp(1.6rem, 4vw, 2.2rem);
            background: var(--primary-accent);
            color: #ffffff;
            padding: 1.8rem;
            cursor: pointer;
            transition: background 0.3s ease;
            display: flex;
            justify-content: space-between;
            align-items: center;
            font-weight: 700;
            letter-spacing: 0.5px;
            border-bottom: 2px solid var(--secondary-accent);
        }

        .year h2:hover {
            background: var(--secondary-accent);
        }

        .year h2 i {
            font-size: 1.8rem;
            transition: transform 0.3s ease;
            color: rgba(255, 255, 255, 0.8);
        }

        .year.active h2 i {
            transform: rotate(180deg);
        }

        .coming-soon h2 {
            background: var(--bg-card);
            color: var(--text-primary);
            cursor: default;
            border-bottom: 1px solid var(--border-color);
        }

        .coming-soon h2 i {
            color: var(--warning-color);
            animation: pulse 1.5s infinite;
        }

        .coming-soon h2:hover {
            background: var(--bg-card);
        }

        .subjects {
            display: none;
            padding: 2.5rem;
            background-color: var(--bg-card);
            border-bottom-left-radius: 1rem;
            border-bottom-right-radius: 1rem;
            border: 1px solid var(--border-color);
            border-top: none;
            animation: slideDown 0.6s ease forwards;
        }

        .subject {
            margin-bottom: 3rem;
            padding: 2.5rem;
            border: 1px solid var(--border-color);
            border-radius: 1rem;
            background-color: var(--bg-body);
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
        }

        .subject h3 {
            font-family: 'Montserrat', sans-serif;
            margin-bottom: 1.8rem;
            font-size: clamp(1.3rem, 3vw, 1.8rem);
            color: var(--highlight-color);
            border-bottom: 2px solid var(--primary-accent);
            padding-bottom: 1rem;
            letter-spacing: 0.2px;
        }

        .units {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
            gap: 1.8rem;
        }

        .unit {
            background-color: var(--bg-card);
            border-radius: 0.8rem;
            padding: 2rem 1.5rem;
            text-align: center;
            box-shadow: 0 4px 18px rgba(0, 0, 0, 0.15);
            transition: all 0.3s ease;
            color: var(--text-primary);
            display: flex;
            flex-direction: column;
            justify-content: center;
            border: 1px solid var(--border-color);
            position: relative;
            overflow: hidden;
            cursor: pointer;
        }

        .unit::before {
            content: '';
            position: absolute;
            top: 50%;
            left: 50%;
            width: 0;
            height: 0;
            background: radial-gradient(circle, var(--highlight-color) 0%, transparent 70%);
            transform: translate(-50%, -50%);
            opacity: 0;
            transition: all 0.5s ease;
            z-index: 0;
        }

        .unit:hover::before {
            width: 200px;
            height: 200px;
            opacity: 0.1;
        }

        .unit:hover {
            transform: translateY(-8px) scale(1.03);
            border-color: var(--highlight-color);
            box-shadow: 0 8px 25px rgba(0, 0, 0, 0.25);
        }

        .unit-title {
            font-weight: 700;
            font-size: 1.25rem;
            margin-bottom: 0.8rem;
            color: var(--text-primary);
            position: relative;
            z-index: 1;
        }

        .unit-buttons {
            position: relative;
            z-index: 1;
        }

        .unit-buttons a {
            text-decoration: none;
            font-size: 1rem;
            color: var(--primary-accent);
            font-weight: 500;
            transition: color 0.3s ease;
        }

        .unit:hover .unit-buttons a {
            color: var(--highlight-color);
        }

        /* --- About Section --- */
        #about-section {
            text-align: center;
            padding: 3rem 1.5rem;
            display: none;
        }

        .about-content {
            padding: 3.5rem;
        }

        #about-section h2 {
            font-family: 'Montserrat', sans-serif;
            font-size: 2.8rem;
            color: var(--primary-accent);
            margin-bottom: 1.8rem;
            text-shadow: 0 2px 5px rgba(0, 0, 0, 0.2);
        }

        #about-section p {
            max-width: 900px;
            margin: auto;
            font-size: 1.15rem;
            line-height: 1.9;
            color: var(--text-secondary);
        }

        #about-section p strong {
            color: var(--highlight-color);
        }

        #back-btn {
            margin-top: 2.5rem;
            padding: 1rem 2.5rem;
            font-size: 1.1rem;
            border: none;
            border-radius: 0.7rem;
            background-color: var(--highlight-color);
            color: #ffffff;
            cursor: pointer;
            transition: transform 0.3s, background-color 0.3s, box-shadow 0.3s;
            font-weight: 600;
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
        }

        #back-btn:hover {
            background-color: #2c9a96;
            transform: translateY(-4px);
            box-shadow: 0 8px 20px rgba(0, 0, 0, 0.3);
        }

        /* --- Footer --- */
        footer {
            text-align: center;
            padding: 2.5rem;
            background-color: var(--bg-card);
            color: var(--text-secondary);
            margin-top: 4rem;
            font-size: 0.95rem;
            border-top: 1px solid var(--border-color);
            box-shadow: 0 -2px 15px rgba(0, 0, 0, 0.1);
        }

        /* --- Scroll to Top Button --- */
        #scrollToTopBtn {
            display: none;
            position: fixed;
            bottom: 30px;
            right: 30px;
            z-index: 999;
            background-color: var(--primary-accent);
            color: white;
            border: none;
            border-radius: 50%;
            width: 50px;
            height: 50px;
            font-size: 1.5rem;
            cursor: pointer;
            box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
            transition: background-color 0.3s ease, transform 0.3s ease;
        }

        #scrollToTopBtn:hover {
            background-color: var(--highlight-color);
            transform: translateY(-3px);
        }

        /* --- Animations --- */
        @keyframes fadeInHeader {
            from { opacity: 0; transform: translateY(-50px); }
            to { opacity: 1; transform: translateY(0); }
        }

        @keyframes headerOverlay {
            0% { transform: rotate(45deg) translate(-50%, -50%); }
            100% { transform: rotate(405deg) translate(-50%, -50%); }
        }

        @keyframes slideDown {
            from { max-height: 0; opacity: 0; transform: translateY(-10px); }
            to { max-height: 1000px; opacity: 1; transform: translateY(0); }
        }

        @keyframes pulse {
            0% { transform: scale(1); opacity: 1; }
            50% { transform: scale(1.1); opacity: 0.7; }
            100% { transform: scale(1); opacity: 1; }
        }

        /* --- Responsive Design --- */
        @media (max-width: 992px) {
            .nav-controls {
                flex-direction: column;
                gap: 1rem;
            }
            .nav-buttons {
                flex-direction: column;
                gap: 0.8rem;
            }
            .nav-buttons button {
                width: 80%;
                margin: auto;
            }
        }

        @media (max-width: 768px) {
            .units {
                grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
            }
            header {
                padding: 2.5rem 1rem;
            }
            header h1 {
                font-size: 2.2rem;
            }
            header p {
                font-size: 1rem;
            }
            .year h2 {
                font-size: 1.5rem;
                padding: 1.2rem;
            }
            .subject h3 {
                font-size: 1.2rem;
            }
            .subject, .about-content {
                padding: 1.5rem;
            }
        }

        @media (max-width: 480px) {
            .units {
                grid-template-columns: 1fr;
            }
            .year h2 {
                font-size: 1.3rem;
            }
            #scrollToTopBtn {
                width: 45px;
                height: 45px;
                font-size: 1.3rem;
                bottom: 20px;
                right: 20px;
            }
        }