* {
            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);
        }

        .timeline {
            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;
        }

        .timeline-title {
            font-size: 1.8em;
            font-weight: bold;
            margin-bottom: 30px;
            text-align: center;
            color: #fbbf24;
        }

        .schedule {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
            gap: 20px;
            margin-bottom: 40px;
        }

        .review-card {
            background: rgba(255, 255, 255, 0.15);
            padding: 25px;
            border-radius: 15px;
            border: 1px solid rgba(255, 255, 255, 0.2);
            text-align: center;
            transition: all 0.3s ease;
        }

        .review-card.due {
            border: 2px solid #f87171;
            background: rgba(248, 113, 113, 0.2);
            animation: pulse 2s infinite;
        }

        .review-card.upcoming {
            border: 2px solid #fbbf24;
            background: rgba(251, 191, 36, 0.2);
        }

        .review-card.future {
            border: 2px solid #4ade80;
            background: rgba(74, 222, 128, 0.2);
        }

        .review-interval {
            font-size: 2em;
            font-weight: bold;
            margin-bottom: 10px;
        }

        .review-label {
            font-size: 0.9em;
            opacity: 0.8;
            margin-bottom: 15px;
        }

        .review-status {
            padding: 5px 15px;
            border-radius: 15px;
            font-size: 0.85em;
            font-weight: bold;
            display: inline-block;
        }

        .status-due {
            background: rgba(248, 113, 113, 0.3);
            color: #f87171;
        }

        .status-upcoming {
            background: rgba(251, 191, 36, 0.3);
            color: #fbbf24;
        }

        .status-future {
            background: rgba(74, 222, 128, 0.3);
            color: #4ade80;
        }

        .content-card {
            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;
        }

        .content-header {
            font-size: 2em;
            font-weight: bold;
            margin-bottom: 10px;
        }

        .mastery-level {
            display: flex;
            align-items: center;
            gap: 15px;
            margin: 20px 0;
            padding: 20px;
            background: rgba(255, 255, 255, 0.1);
            border-radius: 15px;
        }

        .mastery-bar {
            flex: 1;
            height: 30px;
            background: rgba(0, 0, 0, 0.3);
            border-radius: 15px;
            overflow: hidden;
            position: relative;
        }

        .mastery-fill {
            height: 100%;
            background: linear-gradient(90deg, #f87171, #fbbf24, #4ade80);
            transition: width 0.5s ease;
            display: flex;
            align-items: center;
            justify-content: center;
            font-weight: bold;
        }

        .concept-explanation {
            line-height: 1.8;
            opacity: 0.95;
            font-size: 1.1em;
            margin: 25px 0;
        }

        .key-points {
            background: rgba(74, 222, 128, 0.2);
            border: 2px solid rgba(74, 222, 128, 0.5);
            border-radius: 15px;
            padding: 25px;
            margin: 25px 0;
        }

        .key-points h3 {
            color: #4ade80;
            font-size: 1.4em;
            margin-bottom: 15px;
        }

        .key-points ul {
            list-style: none;
            margin-left: 0;
        }

        .key-points li {
            padding: 12px;
            margin: 10px 0;
            background: rgba(255, 255, 255, 0.1);
            border-radius: 10px;
            padding-left: 45px;
            position: relative;
        }

        .key-points li:before {
            content: "✓";
            position: absolute;
            left: 15px;
            color: #4ade80;
            font-weight: bold;
            font-size: 1.2em;
        }

        .quiz-section {
            background: rgba(251, 191, 36, 0.2);
            border: 2px solid rgba(251, 191, 36, 0.5);
            border-radius: 15px;
            padding: 25px;
            margin: 25px 0;
        }

        .quiz-section h3 {
            color: #fbbf24;
            font-size: 1.4em;
            margin-bottom: 15px;
        }

        .action-buttons {
            display: flex;
            gap: 15px;
            justify-content: center;
            margin-top: 30px;
            flex-wrap: wrap;
        }

        .action-btn {
            padding: 15px 40px;
            border: none;
            border-radius: 25px;
            color: white;
            font-size: 1.1em;
            font-weight: bold;
            cursor: pointer;
            transition: all 0.3s ease;
            box-shadow: 0 5px 15px rgba(0, 0, 0, 0.3);
        }

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

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

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

        .action-btn:hover {
            transform: translateY(-2px);
            box-shadow: 0 8px 20px rgba(0, 0, 0, 0.4);
        }

        .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;
            cursor: pointer;
            border: none;
        }

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

        @keyframes pulse {
            0%, 100% { transform: scale(1); }
            50% { transform: scale(1.02); }
        }

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

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