* { margin: 0; padding: 0; box-sizing: border-box; }
        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
            background: linear-gradient(135deg, #0b43aa 0%, #4ba24c 100%);
            min-height: 100vh;
            padding: 20px;
            color: white;
            display: flex;
            flex-direction: column;
            align-items: center;
        }
        .container {
            width: 100%;
            max-width: 500px;
        }
        .header { text-align: center; margin-bottom: 30px; }
        .header h1 { font-size: 2rem; font-weight: 300; margin-bottom: 10px; }
        .stats {
            display: flex;
            justify-content: center;
            gap: 20px;
            margin-bottom: 20px;
            flex-wrap: wrap;
        }
        .stat-box {
            background: rgba(255, 255, 255, 0.2);
            padding: 10px 15px;
            border-radius: 10px;
            backdrop-filter: blur(10px);
            text-align: center;
            min-width: 80px;
        }
        .stat-number { font-size: 1.5rem; font-weight: 600; }
        .stat-label { font-size: 0.8rem; opacity: 0.8; }
        .controls {
            display: flex;
            justify-content: center;
            gap: 10px;
            margin-bottom: 20px;
            flex-wrap: wrap;
        }
        .control-btn {
            background: rgba(255, 255, 255, 0.2);
            border: 1px solid rgba(255, 255, 255, 0.3);
            color: white;
            padding: 8px 16px;
            border-radius: 20px;
            cursor: pointer;
            transition: all 0.3s;
        }
        .control-btn:hover, .control-btn.active {
            background: rgba(255, 255, 255, 0.3);
            transform: translateY(-1px);
        }
        .flashcard {
            width: 400px;
            height: 250px;
            max-width: 90vw;
            margin: 0 auto 20px;
            perspective: 1000px;
        }
        .card-inner {
            position: relative;
            width: 100%;
            height: 100%;
            transition: transform 0.6s;
            transform-style: preserve-3d;
            cursor: pointer;
        }
        .flashcard.flipped .card-inner { transform: rotateY(180deg); }
        .card-front, .card-back {
            position: absolute;
            width: 100%;
            height: 100%;
            -webkit-backface-visibility: hidden;
            backface-visibility: hidden;
            border-radius: 15px;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
            padding: 20px;
            box-shadow: 0 10px 30px rgba(0, 0, 0, 0.2);
        }
        .card-front {
            background: rgba(255, 255, 255, 0.95);
            color: #333;
        }
        .card-back {
            background: rgba(255, 255, 255, 0.98);
            color: #333;
            transform: rotateY(180deg);
        }
        .word {
            font-size: 2rem;
            font-weight: 300;
            margin-bottom: 10px;
            color: #2d3748;
            text-align: center;
        }
        .definition {
            font-size: 1.2rem;
            line-height: 1.4;
            text-align: center;
            margin-bottom: 15px;
        }
        .navigation {
            display: flex;
            justify-content: center;
            gap: 15px;
            margin-bottom: 20px;
        }
        .nav-btn {
            background: rgba(255, 255, 255, 0.9);
            border: none;
            color: #4a5568;
            padding: 10px 20px;
            border-radius: 25px;
            cursor: pointer;
            transition: all 0.3s;
            font-weight: 500;
        }
        .nav-btn:hover { background: white; transform: translateY(-1px); }
        .nav-btn:disabled { opacity: 0.5; cursor: not-allowed; transform: none; }
        .confidence-buttons {
            display: flex;
            gap: 10px;
            margin-top: 15px;
            opacity: 0;
            transition: opacity 0.3s;
            pointer-events: none;
        }
        .flashcard.flipped .confidence-buttons { opacity: 1; pointer-events: all; }
        .confidence-btn {
            border: none;
            padding: 8px 16px;
            border-radius: 15px;
            cursor: pointer;
            font-size: 0.85rem;
            font-weight: 500;
            color: white;
            transition: transform 0.2s;
        }
        .confidence-btn:hover { transform: scale(1.05); }
        .confidence-btn.easy { background: #48bb78; }
        .confidence-btn.medium { background: #ed8936; }
        .confidence-btn.hard { background: #e53e3e; }
        .progress-bar {
            width: 100%;
            max-width: 400px;
            height: 6px;
            background: rgba(255, 255, 255, 0.3);
            border-radius: 3px;
            margin: 0 auto 20px;
            overflow: hidden;
        }
        .progress-fill {
            height: 100%;
            background: linear-gradient(90deg, #48bb78, #38a169);
            border-radius: 3px;
            transition: width 0.5s;
            width: 0%;
        }
        @media (max-width: 768px) {
            .flashcard { width: 350px; height: 220px; }
            .word { font-size: 1.8rem; }
            .definition { font-size: 1.1rem; }
        }