:root {
            --primary: #1a2332;
            --secondary: #ff6b6b;
            --accent: #4ecdc4;
            --neutral: #f8f9fa;
            --text: #2c3e50;
        }
        
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        
        body {
            font-family: 'Inter', sans-serif;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            min-height: 100vh;
            color: var(--text);
            overflow-x: hidden;
        }
        
        .hero-section {
            background: url('resources/hero-study.jpg') center/cover;
            position: relative;
            min-height: 40vh;
            display: flex;
            align-items: center;
            justify-content: center;
        }
        
        .hero-overlay {
            position: absolute;
            inset: 0;
            background: rgba(26, 35, 50, 0.7);
        }
        
        .hero-content {
            position: relative;
            z-index: 10;
            text-align: center;
            color: white;
            max-width: 800px;
            padding: 2rem;
        }
        
        .hero-title {
            font-family: 'Playfair Display', serif;
            font-size: 3.5rem;
            font-weight: 700;
            margin-bottom: 1rem;
            background: linear-gradient(45deg, #ff6b6b, #4ecdc4);
            -webkit-background-clip: text;
            -webkit-text-fill-color: transparent;
            background-clip: text;
        }
        
        .hero-subtitle {
            font-size: 1.25rem;
            margin-bottom: 2rem;
            opacity: 0.9;
        }
        
        .navigation {
            background: rgba(255, 255, 255, 0.95);
            backdrop-filter: blur(10px);
            border-bottom: 1px solid rgba(255, 255, 255, 0.2);
            position: sticky;
            top: 0;
            z-index: 50;
        }
        
        .nav-container {
            max-width: 1200px;
            margin: 0 auto;
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding: 1rem 2rem;
        }
        
        .logo {
            display: flex;
            align-items: center;
            gap: 0.5rem;
            font-weight: 700;
            font-size: 1.5rem;
            color: var(--primary);
        }
        
        .logo-icon {
            width: 40px;
            height: 40px;
            background: var(--secondary);
            border-radius: 8px;
            display: flex;
            align-items: center;
            justify-content: center;
            color: white;
            font-size: 1.2rem;
        }
        
        .nav-links {
            display: flex;
            gap: 2rem;
            list-style: none;
        }
        
        .nav-link {
            color: var(--text);
            text-decoration: none;
            font-weight: 500;
            padding: 0.5rem 1rem;
            border-radius: 8px;
            transition: all 0.3s ease;
        }
        
        .nav-link:hover, .nav-link.active {
            background: var(--accent);
            color: white;
            transform: translateY(-2px);
        }
        
        .main-content {
            max-width: 1200px;
            margin: 0 auto;
            padding: 2rem;
        }
        
        .stats-grid {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
            gap: 1.5rem;
            margin-bottom: 3rem;
        }
        
        .stats-card {
            background: white;
            padding: 2rem;
            border-radius: 16px;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
            text-align: center;
            transition: transform 0.3s ease;
        }
        
        .stats-card:hover {
            transform: translateY(-5px);
        }
        
        .stats-number {
            font-size: 2.5rem;
            font-weight: 700;
            color: var(--secondary);
            margin-bottom: 0.5rem;
        }
        
        .stats-label {
            color: #666;
            font-weight: 500;
        }
        
        .study-interface {
            background: white;
            border-radius: 20px;
            padding: 2rem;
            box-shadow: 0 20px 40px rgba(0, 0, 0, 0.1);
            margin-bottom: 2rem;
        }
        
        .mode-selector {
            display: flex;
            gap: 1rem;
            margin-bottom: 2rem;
            flex-wrap: wrap;
        }
        
        .mode-btn {
            padding: 0.75rem 1.5rem;
            border: 2px solid var(--accent);
            background: transparent;
            color: var(--accent);
            border-radius: 25px;
            font-weight: 600;
            cursor: pointer;
            transition: all 0.3s ease;
        }
        
        .mode-btn:hover, .mode-btn.active {
            background: var(--accent);
            color: white;
            transform: translateY(-2px);
        }
        
        .progress-section {
            margin-bottom: 2rem;
        }
        
        .progress-bar-container {
            background: #e9ecef;
            border-radius: 10px;
            height: 8px;
            margin-bottom: 1rem;
            overflow: hidden;
        }
        
        .progress-bar {
            height: 100%;
            background: linear-gradient(90deg, var(--accent), var(--secondary));
            border-radius: 10px;
            transition: width 0.5s ease;
            width: 0%;
        }
        
        .progress-info {
            display: flex;
            justify-content: space-between;
            align-items: center;
            font-weight: 500;
        }
        
        .flashcard-container {
            perspective: 1000px;
            margin: 2rem 0;
            display: flex;
            justify-content: center;
        }
        
        .flashcard {
            width: 400px;
            height: 300px;
            position: relative;
            transform-style: preserve-3d;
            cursor: pointer;
            transition: transform 0.6s;
        }
        
        .flashcard-front, .flashcard-back {
            position: absolute;
            width: 100%;
            height: 100%;
            backface-visibility: hidden;
            border-radius: 16px;
            padding: 2rem;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            text-align: center;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
        }
        
        .flashcard-front {
            background: linear-gradient(135deg, var(--accent), var(--secondary));
            color: white;
        }
        
        .flashcard-back {
            background: white;
            color: var(--text);
            transform: rotateY(180deg);
        }
        
        .word-level {
            position: absolute;
            top: 1rem;
            right: 1rem;
            padding: 0.25rem 0.75rem;
            border-radius: 12px;
            font-size: 0.8rem;
            font-weight: 600;
        }
        
        .level-1 { background: rgba(255, 255, 255, 0.2); }
        .level-2 { background: rgba(255, 255, 255, 0.3); }
        .level-3 { background: rgba(255, 255, 255, 0.4); }
        .level-4 { background: rgba(255, 255, 255, 0.5); }
        
        .word-text {
            font-family: 'Playfair Display', serif;
            font-size: 2.5rem;
            font-weight: 700;
            margin-bottom: 1rem;
        }
        
        .pronunciation {
            font-size: 1.1rem;
            opacity: 0.9;
            margin-bottom: 1rem;
        }
        
        .flip-hint {
            font-size: 0.9rem;
            opacity: 0.7;
        }
        
        .definition {
            font-size: 1.3rem;
            font-weight: 600;
            margin-bottom: 1.5rem;
            line-height: 1.4;
        }
        
        .example {
            background: var(--neutral);
            padding: 1rem;
            border-radius: 8px;
            margin-bottom: 1rem;
            font-style: italic;
        }
        
        .synonyms {
            font-size: 0.9rem;
            color: #666;
        }
        
        .controls {
            display: flex;
            justify-content: center;
            gap: 1rem;
            margin-top: 2rem;
            flex-wrap: wrap;
        }
        
        .control-btn {
            padding: 0.75rem 1.5rem;
            border: none;
            border-radius: 8px;
            font-weight: 600;
            cursor: pointer;
            transition: all 0.3s ease;
            display: flex;
            align-items: center;
            gap: 0.5rem;
        }
        
        .btn-primary {
            background: var(--primary);
            color: white;
        }
        
        .btn-secondary {
            background: var(--secondary);
            color: white;
        }
        
        .btn-accent {
            background: var(--accent);
            color: white;
        }
        
        .control-btn:hover {
            transform: translateY(-2px);
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
        }
        
        .notification {
            position: fixed;
            top: 2rem;
            right: 2rem;
            padding: 1rem 1.5rem;
            border-radius: 8px;
            color: white;
            font-weight: 600;
            z-index: 1000;
        }
        
        .notification-success { background: var(--accent); }
        .notification-warning { background: #f39c12; }
        .notification-error { background: #e74c3c; }
        .notification-info { background: var(--primary); }
        
        .particles-canvas {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            pointer-events: none;
            z-index: 1;
        }
        
        @media (max-width: 768px) {
            .hero-title {
                font-size: 2.5rem;
            }
            
            .flashcard {
                width: 350px;
                height: 250px;
            }
            
            .word-text {
                font-size: 2rem;
            }
            
            .nav-links {
                gap: 1rem;
            }
            
            .nav-container {
                padding: 1rem;
            }
        }
        
        .achievement-showcase {
            background: white;
            border-radius: 16px;
            padding: 2rem;
            margin-top: 2rem;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
        }
        
        .achievement-grid {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
            gap: 1rem;
            margin-top: 1rem;
        }
        
        .achievement-item {
            text-align: center;
            padding: 1rem;
            border-radius: 12px;
            background: var(--neutral);
            transition: transform 0.3s ease;
        }
        
        .achievement-item:hover {
            transform: translateY(-3px);
        }
        
        .achievement-icon {
            font-size: 2rem;
            margin-bottom: 0.5rem;
        }
        
        .footer {
            background: var(--primary);
            color: white;
            text-align: center;
            padding: 2rem;
            margin-top: 4rem;
        }