:root {
            --primary-color: #2c3e50;
            --secondary-color: #3498db;
            --danger-color: #e74c3c;
            --success-color: #2ecc71;
            --warning-color: #f39c12;
            --light-color: #ecf0f1;
            --dark-color: #34495e;
        }
        
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
        }
        
        body {
            background-color: #f5f7fa;
            color: var(--dark-color);
            line-height: 1.6;
        }
        
        .container {
            max-width: 1200px;
            margin: 0 auto;
            padding: 20px;
        }
        
        header {
            background-color: var(--primary-color);
            color: white;
            padding: 1.5rem 0;
            text-align: center;
            border-radius: 8px 8px 0 0;
            margin-bottom: 20px;
        }
        
        h1 {
            font-size: 2.5rem;
            margin-bottom: 10px;
        }
        
        .subtitle {
            font-size: 1.2rem;
            opacity: 0.8;
        }
        
        .dashboard {
            display: grid;
            grid-template-columns: 1fr 3fr;
            gap: 20px;
            margin-bottom: 20px;
        }
        
        .sidebar {
            background-color: white;
            border-radius: 8px;
            padding: 20px;
            box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
        }
        
        .main-content {
            background-color: white;
            border-radius: 8px;
            padding: 20px;
            box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
        }
        
        .nav-list {
            list-style: none;
        }
        
        .nav-item {
            margin-bottom: 10px;
        }
        
        .nav-link {
            display: block;
            padding: 12px 15px;
            background-color: var(--light-color);
            color: var(--dark-color);
            text-decoration: none;
            border-radius: 5px;
            transition: all 0.3s ease;
        }
        
        .nav-link:hover {
            background-color: var(--secondary-color);
            color: white;
        }
        
        .nav-link.active {
            background-color: var(--secondary-color);
            color: white;
        }
        
        .difficulty-badge {
            display: inline-block;
            padding: 3px 8px;
            border-radius: 12px;
            font-size: 0.8rem;
            margin-left: 10px;
        }
        
        .easy {
            background-color: var(--success-color);
            color: white;
        }
        
        .medium {
            background-color: var(--warning-color);
            color: white;
        }
        
        .hard {
            background-color: var(--danger-color);
            color: white;
        }
        
        .challenge-section {
            display: none;
        }
        
        .challenge-section.active {
            display: block;
        }
        
        .challenge-title {
            font-size: 1.8rem;
            margin-bottom: 15px;
            color: var(--primary-color);
            border-bottom: 2px solid var(--light-color);
            padding-bottom: 10px;
        }
        
        .challenge-description {
            margin-bottom: 20px;
            line-height: 1.8;
        }
        
        .code-container {
            background-color: #2d2d2d;
            color: #f8f8f2;
            border-radius: 5px;
            padding: 15px;
            margin-bottom: 20px;
            overflow-x: auto;
            font-family: 'Courier New', monospace;
        }
        
        .input-group {
            margin-bottom: 20px;
        }
        
        .input-label {
            display: block;
            margin-bottom: 8px;
            font-weight: bold;
        }
        
        .input-field {
            width: 100%;
            padding: 10px;
            border: 1px solid #ddd;
            border-radius: 4px;
            font-size: 1rem;
        }
        
        .btn {
            padding: 10px 20px;
            border: none;
            border-radius: 4px;
            cursor: pointer;
            font-size: 1rem;
            transition: background-color 0.3s ease;
        }
        
        .btn-primary {
            background-color: var(--secondary-color);
            color: white;
        }
        
        .btn-primary:hover {
            background-color: #2980b9;
        }
        
        .btn-danger {
            background-color: var(--danger-color);
            color: white;
        }
        
        .btn-danger:hover {
            background-color: #c0392b;
        }
        
        .result-container {
            margin-top: 20px;
            padding: 15px;
            border-radius: 5px;
            background-color: var(--light-color);
            display: none;
        }
        
        .result-container.success {
            display: block;
            background-color: #d4edda;
            color: #155724;
            border: 1px solid #c3e6cb;
        }
        
        .result-container.error {
            display: block;
            background-color: #f8d7da;
            color: #721c24;
            border: 1px solid #f5c6cb;
        }
        
        .hint-container {
            margin-top: 15px;
            padding: 10px;
            background-color: #fff3cd;
            border: 1px solid #ffeaa7;
            border-radius: 4px;
            display: none;
        }
        
        .hint-btn {
            background: none;
            border: none;
            color: var(--warning-color);
            cursor: pointer;
            text-decoration: underline;
            font-size: 0.9rem;
        }
        
        .progress-container {
            margin-top: 20px;
            background-color: var(--light-color);
            border-radius: 5px;
            height: 10px;
            overflow: hidden;
        }
        
        .progress-bar {
            height: 100%;
            background-color: var(--success-color);
            width: 0%;
            transition: width 0.5s ease;
        }
        
        footer {
            text-align: center;
            margin-top: 30px;
            padding: 20px;
            color: #7f8c8d;
            font-size: 0.9rem;
        }