* {
    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;
    display: flex;
    justify-content: center;
    align-items: center;
    color: #333;
}

.container {
    max-width: 600px;
    width: 90%;
    text-align: center;
}

header {
    margin-bottom: 30px;
}

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

header p {
    color: rgba(255,255,255,0.9);
    font-size: 1.1rem;
}

.series-selector {
    display: flex;
    gap: 10px;
    justify-content: center;
    margin-bottom: 25px;
}

.series-btn {
    padding: 10px 20px;
    border: none;
    border-radius: 20px;
    cursor: pointer;
    font-weight: 600;
    transition: all 0.3s ease;
    background: rgba(255,255,255,0.2);
    color: white;
}

.series-btn.active {
    background: white;
    color: #333;
}

.series-btn:hover {
    transform: translateY(-2px);
}

.stats {
    display: flex;
    justify-content: space-between;
    margin-bottom: 30px;
    color: white;
    font-weight: bold;
}

.flashcard {
    margin-bottom: 30px;
}

.card-content {
    width: 100%;
    height: 350px;
    border-radius: 20px;
    box-shadow: 0 10px 30px rgba(0,0,0,0.3);
    background: linear-gradient(135deg, #ffffff 0%, #f8f9fa 100%);
    border: 3px solid #e9ecef;
    display: flex;
    flex-direction: column;
    justify-content: space-evenly;
    align-items: center;
    padding: 30px;
    text-align: center;
}

.lowercase, .uppercase {
    font-size: 4rem;
    font-weight: bold;
    color: #495057;
}

.uppercase {
    font-size: 5rem;
}

.letter-name {
    font-size: 2rem;
    color: #495057;
    font-weight: 600;
}

.pronunciation {
    font-size: 1.8rem;
    color: #28a745;
    font-weight: bold;
    font-style: italic;
}

.description {
    font-size: 1.2rem;
    opacity: 0.9;
    font-style: italic;
}

.controls {
    display: flex;
    gap: 15px;
    justify-content: center;
    margin-bottom: 25px;
}

.btn {
    padding: 12px 24px;
    border: none;
    border-radius: 25px;
    cursor: pointer;
    font-size: 1rem;
    font-weight: 600;
    transition: all 0.3s ease;
    box-shadow: 0 4px 15px rgba(0,0,0,0.2);
}

.btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(0,0,0,0.3);
}

.btn.primary {
    background: linear-gradient(135deg, #007bff 0%, #0056b3 100%);
    color: white;
}

.btn.secondary {
    background: linear-gradient(135deg, #6c757d 0%, #495057 100%);
    color: white;
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
    transform: none;
}

.difficulty-buttons {
    display: flex;
    gap: 10px;
    justify-content: center;
    margin-bottom: 25px;
}

.difficulty-btn {
    padding: 8px 16px;
    border: none;
    border-radius: 20px;
    cursor: pointer;
    font-weight: 600;
    transition: all 0.3s ease;
    opacity: 0.7;
}

.difficulty-btn:hover, .difficulty-btn.active {
    opacity: 1;
    transform: scale(1.05);
}

.difficulty-btn.easy {
    background: #28a745;
    color: white;
}

.difficulty-btn.medium {
    background: #ffc107;
    color: #212529;
}

.difficulty-btn.hard {
    background: #dc3545;
    color: white;
}

.options {
    display: flex;
    gap: 15px;
    justify-content: center;
}

@media (max-width: 768px) {
    header h1 {
        font-size: 2rem;
    }
    
    .lowercase, .uppercase {
        font-size: 3rem;
    }
    
    .uppercase {
        font-size: 3.5rem;
    }
    
    .pronunciation {
        font-size: 2rem;
    }
    
    .controls {
        flex-wrap: wrap;
    }
    
    .difficulty-buttons {
        flex-wrap: wrap;
    }
}