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

        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            min-height: 100vh;
            padding: 20px;
            color: #fff;
        }

        .container {
            max-width: 1200px;
            margin: 0 auto;
        }

        .header {
            text-align: center;
            margin-bottom: 30px;
            padding: 30px;
            background: rgba(255, 255, 255, 0.1);
            backdrop-filter: blur(10px);
            border-radius: 20px;
            border: 1px solid rgba(255, 255, 255, 0.2);
        }

        .header h1 {
            font-size: 2.5em;
            margin-bottom: 10px;
            text-shadow: 2px 2px 4px rgba(0, 0, 0, 0.3);
        }

        .header p {
            font-size: 1.2em;
            opacity: 0.9;
        }

        .level-selector {
            background: rgba(255, 255, 255, 0.1);
            backdrop-filter: blur(10px);
            border-radius: 20px;
            border: 1px solid rgba(255, 255, 255, 0.2);
            padding: 30px;
            margin-bottom: 30px;
        }

        .selector-header {
            text-align: center;
            font-size: 1.5em;
            margin-bottom: 20px;
            font-weight: bold;
        }

        .level-track {
            display: flex;
            align-items: center;
            gap: 10px;
            margin-bottom: 20px;
        }

        .level-number {
            min-width: 40px;
            text-align: center;
            font-size: 1.5em;
            font-weight: bold;
            color: #fbbf24;
        }

        .slider-container {
            flex: 1;
            position: relative;
            height: 60px;
        }

        .slider {
            width: 100%;
            height: 12px;
            border-radius: 10px;
            background: linear-gradient(90deg,
                #4ade80 0%,
                #fbbf24 50%,
                #f87171 100%);
            outline: none;
            -webkit-appearance: none;
            appearance: none;
            cursor: pointer;
            position: absolute;
            top: 50%;
            transform: translateY(-50%);
        }

        .slider::-webkit-slider-thumb {
            -webkit-appearance: none;
            appearance: none;
            width: 30px;
            height: 30px;
            border-radius: 50%;
            background: white;
            cursor: pointer;
            box-shadow: 0 0 10px rgba(0, 0, 0, 0.3), 0 0 20px rgba(255, 255, 255, 0.5);
        }

        .slider::-moz-range-thumb {
            width: 30px;
            height: 30px;
            border-radius: 50%;
            background: white;
            cursor: pointer;
            border: none;
            box-shadow: 0 0 10px rgba(0, 0, 0, 0.3), 0 0 20px rgba(255, 255, 255, 0.5);
        }

        .level-labels {
            display: flex;
            justify-content: space-between;
            margin-top: 10px;
            font-size: 0.85em;
            opacity: 0.8;
        }

        .current-level-display {
            text-align: center;
            margin-top: 20px;
            padding: 20px;
            background: rgba(255, 255, 255, 0.15);
            border-radius: 15px;
        }

        .level-description {
            font-size: 1.1em;
            margin-top: 10px;
        }

        .content-area {
            background: rgba(255, 255, 255, 0.1);
            backdrop-filter: blur(10px);
            border-radius: 20px;
            border: 1px solid rgba(255, 255, 255, 0.2);
            padding: 40px;
            margin-bottom: 30px;
            animation: fadeIn 0.5s ease;
            min-height: 400px;
        }

        .loading {
            text-align: center;
            padding: 60px 20px;
        }

        .loading-spinner {
            border: 4px solid rgba(255, 255, 255, 0.3);
            border-top: 4px solid white;
            border-radius: 50%;
            width: 50px;
            height: 50px;
            animation: spin 1s linear infinite;
            margin: 0 auto 20px;
        }

        @keyframes spin {
            0% { transform: rotate(0deg); }
            100% { transform: rotate(360deg); }
        }

        .content-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            margin-bottom: 30px;
            padding-bottom: 20px;
            border-bottom: 2px solid rgba(255, 255, 255, 0.3);
        }

        .content-title {
            font-size: 2em;
            font-weight: bold;
        }

        .level-badge {
            padding: 10px 25px;
            border-radius: 25px;
            font-weight: bold;
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.3);
        }

        .level-badge.easy {
            background: linear-gradient(135deg, #4ade80, #22c55e);
        }

        .level-badge.medium {
            background: linear-gradient(135deg, #fbbf24, #f59e0b);
        }

        .level-badge.hard {
            background: linear-gradient(135deg, #f87171, #ef4444);
        }

        .explanation-section {
            margin-bottom: 30px;
        }

        .section-title {
            font-size: 1.4em;
            font-weight: bold;
            margin-bottom: 15px;
            color: #fbbf24;
        }

        .section-content {
            line-height: 1.8;
            opacity: 0.95;
            font-size: 1.05em;
        }

        .example-box {
            background: rgba(74, 222, 128, 0.2);
            border: 2px solid rgba(74, 222, 128, 0.5);
            border-radius: 15px;
            padding: 20px;
            margin: 20px 0;
        }

        .example-box h4 {
            color: #4ade80;
            margin-bottom: 10px;
            font-size: 1.2em;
        }

        .analogy-box {
            background: rgba(251, 191, 36, 0.2);
            border: 2px solid rgba(251, 191, 36, 0.5);
            border-radius: 15px;
            padding: 20px;
            margin: 20px 0;
        }

        .analogy-box h4 {
            color: #fbbf24;
            margin-bottom: 10px;
            font-size: 1.2em;
        }

        .key-points {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
            gap: 15px;
            margin-top: 20px;
        }

        .key-point {
            background: rgba(255, 255, 255, 0.15);
            padding: 20px;
            border-radius: 15px;
            border-left: 4px solid #4ade80;
        }

        .key-point h5 {
            font-size: 1.1em;
            margin-bottom: 10px;
            color: #4ade80;
        }

        .navigation {
            display: flex;
            justify-content: space-between;
            gap: 15px;
            margin-top: 30px;
        }

        .nav-btn {
            padding: 15px 30px;
            background: rgba(255, 255, 255, 0.2);
            border: 2px solid rgba(255, 255, 255, 0.3);
            border-radius: 15px;
            color: white;
            text-decoration: none;
            font-size: 1em;
            transition: all 0.3s ease;
            backdrop-filter: blur(10px);
            display: flex;
            align-items: center;
            gap: 10px;
        }

        .nav-btn:hover {
            background: rgba(255, 255, 255, 0.3);
            transform: translateY(-2px);
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.3);
        }

        .action-bar {
            display: flex;
            justify-content: center;
            gap: 15px;
            flex-wrap: wrap;
        }

        #loading {
            text-align: center;
            padding: 40px;
            font-size: 1.2em;
        }

        .error {
            background: rgba(239, 68, 68, 0.2);
            border: 2px solid rgba(239, 68, 68, 0.5);
            color: white;
        }

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

        @media (max-width: 768px) {
            .key-points {
                grid-template-columns: 1fr;
            }

            .content-header {
                flex-direction: column;
                align-items: flex-start;
                gap: 15px;
            }

            .navigation {
                flex-direction: column;
            }

            .action-bar {
                flex-direction: column;
            }
        }