/*       :root {
            --primary: #3498db;
            --primary-dark: #2980b9;
            --secondary: #2ecc71;
            --dark: #34495e;
            --light: #ecf0f1;
            --danger: #e74c3c;
        }*/
        :root {
            --bs-primary:   #2C3E50;
            --bs-secondary: #18BC9C;
            --bs-success:   #1ABC9C;
            --bs-danger:    #E74C3C;
            --bs-warning:   #F39C12;
            --bs-light:     #ECF0F1;
            --bs-dark:      #2D3436;
          }
        * {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
        }
        
        body {
            background-color: #f5f7fa;
            color: #333;
            line-height: 1.6;
        }
        
        header {
            background: var(--bs-dark);
            color: white;
            padding: 1rem 0;
            box-shadow: 0 2px 5px rgba(0,0,0,0.1);
        }
        
        .container {
            width: 90%;
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 20px;
        }
        
        nav {
            display: flex;
            justify-content: space-between;
            align-items: center;
        }
        
        .logo {
            font-size: 1.5rem;
            font-weight: bold;
        }
        
        .main-nav {
            display: flex;
            list-style: none;
        }
        
        .main-nav li {
            margin-left: 20px;
        }
        
        .main-nav a {
            color: white;
            text-decoration: none;
            font-weight: 500;
            transition: color 0.3s;
        }
        
        .main-nav a:hover {
            color: var(--bs-secondary);
        }
        
        main {
            padding: 2rem 0;
        }
        
        .card {
            background: white;
            border-radius: 8px;
            box-shadow: 0 3px 10px rgba(0,0,0,0.1);
            padding: 20px;
            margin-bottom: 20px;
        }
        
        h1, h2, h3 {
            margin-bottom: 1rem;
            color: var(--bs-dark);
        }
        
        .categories {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
            gap: 20px;
            margin-top: 20px;
        }
        
        .category {
            background: white;
            border-radius: 8px;
            box-shadow: 0 3px 10px rgba(0,0,0,0.1);
            padding: 20px;
            transition: transform 0.3s, box-shadow 0.3s;
            cursor: pointer;
        }
        
        .category:hover {
            transform: translateY(-5px);
            box-shadow: 0 5px 15px rgba(0,0,0,0.15);
        }
        
        .category h3 {
            color: var(--bs-primary);
            margin-bottom: 10px;
        }
        
        .btn {
            display: inline-block;
            background: var(--bs-primary);
            color: white;
            padding: 10px 15px;
            border: none;
            border-radius: 4px;
            cursor: pointer;
            text-decoration: none;
            font-size: 1rem;
            margin-top: 10px;
            transition: background 0.3s;
        }
        
        .btn:hover {
            background: var(--bs-dark);
        }
        
        .level-select {
            margin-bottom: 20px;
            display: flex;
            gap: 10px;
        }
        
        .level-btn {
            padding: 8px 15px;
            border: 2px solid var(--bs-primary);
            background: white;
            color: var(--bs-primary);
            border-radius: 4px;
            cursor: pointer;
            font-weight: bold;
        }
        
        .level-btn.active {
            background: var(--bs-primary);
            color: white;
        }
        
        .question-card {
            display: none;
            background: white;
            border-radius: 8px;
            box-shadow: 0 3px 10px rgba(0,0,0,0.1);
            padding: 20px;
            margin-top: 20px;
        }
        
        .question-card.active {
            display: block;
        }
        
        .options {
            margin-top: 15px;
        }
        
        .option {
            display: block;
            padding: 10px 15px;
            margin-bottom: 10px;
            background: var(--bs-light);
            border-radius: 4px;
            cursor: pointer;
            transition: background 0.3s;
        }
        
        .option:hover {
            background: #d5dbdb;
        }
        
        .option.selected {
            background: var(--bs-primary);
            color: white;
        }
        
        .option.correct {
            background: var(--bs-secondary);
            color: white;
        }
        
        .option.incorrect {
            background: var(--bs-danger);
            color: white;
        }
        
        .feedback {
            margin-top: 20px;
            padding: 15px;
            border-radius: 4px;
            display: none;
        }
        
        .feedback.correct {
            background: #d4edda;
            color: #155724;
            display: block;
        }
        
        .feedback.incorrect {
            background: #f8d7da;
            color: #721c24;
            display: block;
        }
        
        .controls {
            margin-top: 20px;
            display: flex;
            justify-content: space-between;
        }
        
        .progress-bar {
            height: 10px;
            background: #e0e0e0;
            border-radius: 5px;
            margin: 20px 0;
            overflow: hidden;
        }
        
        .progress {
            height: 100%;
            background: var(--bs-primary);
            width: 0%;
            transition: width 0.5s;
        }
        
        /* Fade-in animation for question transitions */
        @keyframes fadeIn {
          from { opacity: 0; }
          to { opacity: 1; }
        }
        .fade-in {
          animation: fadeIn 0.3s ease-in-out;
        }
        
        @media (max-width: 768px) {
            .categories {
                grid-template-columns: 1fr;
            }
            
            .main-nav {
                display: none;
            }
        }