:root {
            --apple-white: #f5f5f7;
            --apple-light-gray: #d2d2d7;
            --apple-gray: #86868b;
            --apple-dark-gray: #424245;
            --apple-black: #1d1d1f;
            --apple-blue: #0071e3;
            --apple-blue-light: #2997ff;
            --apple-blue-lighter: #e8f2ff;
            --apple-green: #00c04d;
            --apple-red: #ff3b30;
            --apple-purple: #bf5af2;
            
            --text-primary: #1d1d1f;
            --text-secondary: #424245;
            --text-tertiary: #86868b;
            
            --surface-primary: #ffffff;
            --surface-secondary: #f5f5f7;
            --surface-tertiary: #e8e8ed;
            
            --border-radius: 18px;
            --border-radius-small: 10px;
            --border-radius-large: 22px;
            --transition: all 0.3s cubic-bezier(0.25, 0.1, 0.25, 1);
            --transition-slow: all 0.5s cubic-bezier(0.25, 0.1, 0.25, 1);
            --shadow-subtle: 0 4px 24px rgba(0, 0, 0, 0.04);
            --shadow-medium: 0 8px 32px rgba(0, 0, 0, 0.08);
            --shadow-float: 0 16px 40px rgba(0, 0, 0, 0.1);
        }

        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'SF Pro Display', sans-serif;
        }

        html {
            scroll-behavior: smooth;
        }

        body {
            background-color: var(--surface-primary);
            color: var(--text-primary);
            min-height: 100vh;
            position: relative;
            overflow-x: hidden;
            -webkit-font-smoothing: antialiased;
            -moz-osx-font-smoothing: grayscale;
        }

        /* Page Transition Styles */
        .page {
            position: absolute;
            width: 100%;
            height: 100vh;
            display: flex;
            flex-direction: column;
            transition: var(--transition-slow);
            z-index: 10;
            background-color: var(--surface-primary);
            opacity: 1;
            transform: translateY(0);
            overflow: hidden;
        }

        .page.hidden {
            opacity: 0;
            pointer-events: none;
            transform: translateY(20px);
        }

        /* Navigation Bar */
        .nav-bar {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 64px;
            background-color: rgba(255, 255, 255, 0.8);
            backdrop-filter: saturate(180%) blur(20px);
            -webkit-backdrop-filter: saturate(180%) blur(20px);
            display: flex;
            align-items: center;
            justify-content: space-between;
            padding: 0 24px;
            z-index: 1000;
            border-bottom: 1px solid rgba(0, 0, 0, 0.05);
            flex-shrink: 0;
        }

        .nav-logo {
            display: flex;
            align-items: center;
            gap: 8px;
            font-weight: 600;
            font-size: 1.2rem;
            color: var(--text-primary);
            text-decoration: none;
        }

        .nav-logo-icon {
            font-size: 1.4rem;
            color: var(--apple-blue);
        }

        .nav-links {
            display: flex;
            gap: 32px;
        }

        .nav-link {
            font-size: 0.95rem;
            font-weight: 400;
            color: var(--text-secondary);
            text-decoration: none;
            transition: var(--transition);
            position: relative;
            padding: 8px 0;
        }

        .nav-link:hover {
            color: var(--text-primary);
        }

        .nav-link.active {
            color: var(--apple-blue);
        }

        .nav-link.active::after {
            content: '';
            position: absolute;
            bottom: 0;
            left: 0;
            width: 100%;
            height: 2px;
            background-color: var(--apple-blue);
            border-radius: 1px;
        }

        .nav-actions {
            display: flex;
            align-items: center;
            gap: 16px;
        }

        .nav-action-btn {
            background: none;
            border: none;
            font-size: 1.1rem;
            color: var(--text-secondary);
            cursor: pointer;
            transition: var(--transition);
            display: flex;
            align-items: center;
            justify-content: center;
            width: 36px;
            height: 36px;
            border-radius: 50%;
        }

        .nav-action-btn:hover {
            background-color: var(--surface-secondary);
            color: var(--text-primary);
        }

        /* Back Button */
        .back-btn {
            position: fixed;
            top: 76px;
            left: 24px;
            background: rgba(255, 255, 255, 0.9);
            backdrop-filter: saturate(180%) blur(10px);
            -webkit-backdrop-filter: saturate(180%) blur(10px);
            border: 1px solid rgba(0, 0, 0, 0.05);
            font-size: 0.95rem;
            font-weight: 400;
            color: var(--text-secondary);
            cursor: pointer;
            display: flex;
            align-items: center;
            gap: 8px;
            padding: 10px 16px;
            border-radius: var(--border-radius-small);
            transition: var(--transition);
            z-index: 999;
            box-shadow: var(--shadow-subtle);
        }

        .back-btn:hover {
            color: var(--apple-blue);
            background-color: rgba(255, 255, 255, 0.95);
            transform: translateX(-2px);
        }

        .back-btn i {
            font-size: 0.9rem;
        }

        /* Main Page - Centered Layout */
        .main-content {
            flex: 1;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            text-align: center;
            padding: 0 24px;
            max-width: 1200px;
            margin: 0 auto;
            width: 100%;
        }

        .logo-container {
            margin-bottom: 48px;
            animation: fadeInUp 0.8s ease-out;
        }

        .logo {
            font-size: 3.8rem;
            font-weight: 700;
            margin-bottom: 16px;
            background: linear-gradient(135deg, var(--apple-black) 0%, var(--apple-blue) 100%);
            -webkit-background-clip: text;
            -webkit-text-fill-color: transparent;
            background-clip: text;
            letter-spacing: -0.5px;
            line-height: 1.05;
        }

        .tagline {
            font-size: 1.8rem;
            font-weight: 500;
            margin-bottom: 24px;
            color: var(--text-secondary);
            line-height: 1.4;
            max-width: 700px;
            animation: fadeInUp 0.8s ease-out 0.2s forwards;
            opacity: 0;
        }

        .sub-tagline {
            font-size: 1.1rem;
            color: var(--text-tertiary);
            margin-bottom: 48px;
            max-width: 500px;
            line-height: 1.7;
            animation: fadeInUp 0.8s ease-out 0.4s forwards;
            opacity: 0;
        }

        /* Button Styles */
        .btn {
            display: inline-flex;
            align-items: center;
            justify-content: center;
            padding: 16px 36px;
            background-color: var(--apple-blue);
            color: white;
            border-radius: var(--border-radius-large);
            font-weight: 500;
            font-size: 1.05rem;
            border: none;
            cursor: pointer;
            min-width: 180px;
            transition: var(--transition);
            box-shadow: 0 4px 16px rgba(0, 113, 227, 0.2);
            animation: fadeInUp 0.8s ease-out 0.6s forwards;
            opacity: 0;
            position: relative;
            overflow: hidden;
        }

        .btn::after {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background-color: rgba(255, 255, 255, 0.1);
            opacity: 0;
            transition: var(--transition);
        }

        .btn:hover {
            background-color: var(--apple-blue-light);
            transform: translateY(-2px);
            box-shadow: 0 8px 24px rgba(0, 113, 227, 0.3);
        }

        .btn:hover::after {
            opacity: 1;
        }

        /* Content Pages - Scrollable Layout */
        .page-content {
            flex: 1;
            display: flex;
            flex-direction: column;
            width: 100%;
            height: calc(100vh - 64px);
            margin-top: 64px;
            overflow-y: auto;
            overflow-x: hidden;
            -webkit-overflow-scrolling: touch;
            scroll-behavior: smooth;
        }

        /* Scrollable Content Container */
        .scrollable-content {
            flex: 1;
            display: flex;
            flex-direction: column;
            min-height: min-content;
            padding: 40px 24px 120px;
            width: 100%;
            max-width: 1200px;
            margin: 0 auto;
        }

        /* Section Titles */
        .section-header {
            width: 100%;
            margin-bottom: 48px;
        }

        .section-title {
            font-size: 2.5rem;
            font-weight: 700;
            margin-bottom: 16px;
            color: var(--text-primary);
            text-align: center;
            line-height: 1.1;
            letter-spacing: -0.3px;
        }

        .section-subtitle {
            font-size: 1.2rem;
            font-weight: 400;
            color: var(--text-secondary);
            text-align: center;
            max-width: 600px;
            margin: 0 auto;
            line-height: 1.6;
        }

        /* Cards Container */
        .cards-container {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
            gap: 24px;
            width: 100%;
            margin: 0 auto;
        }

        /* Subject Cards */
        .subject-card {
            height: 320px;
            border-radius: var(--border-radius);
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            cursor: pointer;
            position: relative;
            overflow: hidden;
            background: var(--surface-secondary);
            border: 1px solid rgba(0, 0, 0, 0.03);
            transition: var(--transition);
            padding: 32px 24px;
            text-decoration: none;
            color: inherit;
        }

        .subject-card::before {
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background: linear-gradient(135deg, rgba(255,255,255,0.1) 0%, rgba(255,255,255,0) 100%);
            opacity: 0;
            transition: var(--transition);
        }

        .subject-card:hover {
            transform: translateY(-8px);
            box-shadow: var(--shadow-float);
            background-color: white;
        }

        .subject-card:hover::before {
            opacity: 1;
        }

        .subject-icon {
            width: 80px;
            height: 80px;
            border-radius: var(--border-radius);
            display: flex;
            align-items: center;
            justify-content: center;
            margin-bottom: 28px;
            background-color: white;
            color: var(--apple-blue);
            font-size: 2rem;
            transition: var(--transition);
            box-shadow: var(--shadow-subtle);
        }

        .subject-card:hover .subject-icon {
            transform: scale(1.05);
            box-shadow: var(--shadow-medium);
        }

        .subject-card.grammar .subject-icon {
            color: var(--apple-purple);
        }

        .subject-name {
            font-size: 1.5rem;
            font-weight: 600;
            color: var(--text-primary);
            margin-bottom: 12px;
            text-align: center;
            line-height: 1.3;
        }

        .subject-desc {
            font-size: 0.95rem;
            color: var(--text-tertiary);
            text-align: center;
            line-height: 1.6;
        }

        /* Level Cards */
        .level-card {
            height: 260px;
            border-radius: var(--border-radius);
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            cursor: pointer;
            position: relative;
            overflow: hidden;
            background: var(--surface-secondary);
            border: 1px solid rgba(0, 0, 0, 0.03);
            transition: var(--transition);
            padding: 32px 24px;
            text-decoration: none;
            color: inherit;
        }

        .level-card:hover {
            transform: translateY(-6px);
            box-shadow: var(--shadow-float);
            background-color: white;
        }

        .level-number {
            width: 60px;
            height: 60px;
            border-radius: 50%;
            display: flex;
            align-items: center;
            justify-content: center;
            margin-bottom: 24px;
            background-color: white;
            color: var(--apple-blue);
            font-size: 1.3rem;
            font-weight: 600;
            transition: var(--transition);
            box-shadow: var(--shadow-subtle);
        }

        .level-card:hover .level-number {
            transform: scale(1.1);
            box-shadow: var(--shadow-medium);
        }

        .level-name {
            font-size: 1.3rem;
            font-weight: 600;
            color: var(--text-primary);
            margin-bottom: 8px;
            text-align: center;
            line-height: 1.3;
        }

        .level-desc {
            font-size: 0.9rem;
            color: var(--text-tertiary);
            text-align: center;
            line-height: 1.5;
        }

        /* Sub-Level Cards */
        .sublevel-card {
            height: 200px;
            border-radius: var(--border-radius);
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            cursor: pointer;
            position: relative;
            overflow: hidden;
            background: var(--surface-secondary);
            border: 1px solid rgba(0, 0, 0, 0.03);
            transition: var(--transition);
            padding: 24px 20px;
            text-decoration: none;
            color: inherit;
        }

        .sublevel-card:hover {
            transform: translateY(-4px);
            box-shadow: var(--shadow-medium);
            background-color: white;
        }

        .sublevel-number {
            width: 44px;
            height: 44px;
            border-radius: 50%;
            display: flex;
            align-items: center;
            justify-content: center;
            margin-bottom: 16px;
            background-color: white;
            color: var(--apple-blue);
            font-size: 1rem;
            font-weight: 600;
            transition: var(--transition);
            box-shadow: var(--shadow-subtle);
        }

        .sublevel-card:hover .sublevel-number {
            transform: scale(1.1);
            box-shadow: var(--shadow-medium);
        }

        .sublevel-name {
            font-size: 1.1rem;
            font-weight: 600;
            color: var(--text-primary);
            margin-bottom: 8px;
            text-align: center;
            line-height: 1.3;
        }

        .sublevel-desc {
            font-size: 0.85rem;
            color: var(--text-tertiary);
            text-align: center;
            line-height: 1.5;
        }

        /* Footer Navigation */
        .footer-nav {
            position: fixed;
            bottom: 0;
            left: 0;
            width: 100%;
            background: rgba(255, 255, 255, 0.8);
            backdrop-filter: saturate(180%) blur(20px);
            -webkit-backdrop-filter: saturate(180%) blur(20px);
            padding: 12px 24px;
            display: flex;
            justify-content: center;
            gap: 20px;
            box-shadow: 0 -2px 20px rgba(0, 0, 0, 0.03);
            z-index: 100;
            border-top: 1px solid rgba(0, 0, 0, 0.05);
        }

        .footer-btn {
            background: none;
            border: none;
            font-size: 0.85rem;
            font-weight: 400;
            color: var(--text-tertiary);
            cursor: pointer;
            padding: 12px 16px;
            border-radius: var(--border-radius-small);
            transition: var(--transition);
            display: flex;
            flex-direction: column;
            align-items: center;
            gap: 6px;
            min-width: 70px;
        }

        .footer-btn i {
            font-size: 1.1rem;
            transition: var(--transition);
        }

        .footer-btn:hover {
            color: var(--apple-blue);
            background-color: rgba(0, 113, 227, 0.05);
        }

        .footer-btn.active {
            color: var(--apple-blue);
            background-color: rgba(0, 113, 227, 0.08);
        }

        .footer-btn.active i {
            transform: translateY(-2px);
        }

        /* Animations */
        @keyframes fadeIn {
            from { opacity: 0; }
            to { opacity: 1; }
        }

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

        @keyframes scaleIn {
            from { 
                opacity: 0;
                transform: scale(0.95);
            }
            to { 
                opacity: 1;
                transform: scale(1);
            }
        }

        /* Responsive Design */
        @media (max-width: 834px) {
            .logo {
                font-size: 3.2rem;
            }
            
            .tagline {
                font-size: 1.5rem;
            }
            
            .section-title {
                font-size: 2.2rem;
            }
            
            .cards-container {
                grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
                gap: 20px;
            }
            
            .nav-links {
                display: none;
            }
            
            .footer-nav {
                justify-content: space-around;
                gap: 0;
                padding: 12px 10px;
            }
            
            .footer-btn {
                min-width: 60px;
                padding: 10px 8px;
                font-size: 0.8rem;
            }
            
            /* Mobile back button adjustments */
            .back-btn {
                top: 72px;
                left: 16px;
                padding: 8px 14px;
                font-size: 0.9rem;
                z-index: 1001;
            }
            
            /* Adjust scrollable content padding */
            .scrollable-content {
                padding: 30px 16px 100px;
            }
            
            .section-header {
                margin-bottom: 36px;
            }
        }

        @media (max-width: 480px) {
            .logo {
                font-size: 2.5rem;
            }
            
            .tagline {
                font-size: 1.3rem;
            }
            
            .sub-tagline {
                font-size: 1rem;
            }
            
            .section-title {
                font-size: 1.9rem;
            }
            
            .section-subtitle {
                font-size: 1.1rem;
                margin-bottom: 32px;
            }
            
            .cards-container {
                grid-template-columns: 1fr;
                max-width: 400px;
                margin: 0 auto;
            }
            
            .btn {
                padding: 14px 28px;
                min-width: 160px;
                font-size: 1rem;
            }
            
            /* Mobile back button fixes */
            .back-btn {
                top: 72px;
                left: 16px;
                padding: 8px 12px;
                font-size: 0.85rem;
                z-index: 1001;
            }
            
            /* Mobile scrollable content adjustments */
            .scrollable-content {
                padding: 25px 16px 90px;
            }
            
            .footer-btn {
                font-size: 0.75rem;
            }
            
            .footer-btn i {
                font-size: 1rem;
            }
            
            .subject-card {
                height: 280px;
            }
            
            .level-card {
                height: 220px;
            }
            
            .sublevel-card {
                height: 180px;
            }
        }
        
        /* Tablet adjustments */
        @media (min-width: 769px) and (max-width: 1024px) {
            .cards-container {
                grid-template-columns: repeat(2, 1fr);
            }
            
            .scrollable-content {
                padding: 40px 32px 120px;
            }
        }
        
        /* Desktop adjustments */
        @media (min-width: 1025px) {
            .cards-container {
                grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
            }
        }
        
        /* Custom scrollbar for webkit browsers */
        .page-content::-webkit-scrollbar {
            width: 8px;
        }
        
        .page-content::-webkit-scrollbar-track {
            background: transparent;
        }
        
        .page-content::-webkit-scrollbar-thumb {
            background: var(--apple-light-gray);
            border-radius: 4px;
        }
        
        .page-content::-webkit-scrollbar-thumb:hover {
            background: var(--apple-gray);
        }