:root {
    --primary: #6366f1;
    --primary-hover: #4f46e5;
    --bg: #f8fafc;
    --card-bg: #ffffff;
    --text-main: #1e293b;
    --text-muted: #64748b;
    --shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.1), 0 8px 10px -6px rgba(0, 0, 0, 0.1);
    --transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

body {
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
    background-color: var(--bg);
    color: var(--text-main);
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
}

.container {
    width: 100%;
    max-width: 500px;
    padding: 2rem;
    text-align: center;
}

h1 {
    font-size: 2rem;
    margin-bottom: 2rem;
    font-weight: 800;
    letter-spacing: -0.025em;
}

.progress-wrapper {
    margin-bottom: 2rem;
}

.progress-bar {
    background: #e2e8f0;
    height: 8px;
    border-radius: 4px;
    overflow: hidden;
    margin-bottom: 0.5rem;
}

.progress-fill {
    background: var(--primary);
    height: 100%;
    width: 0%;
    transition: width 0.4s ease;
}

.progress-text {
    font-size: 0.875rem;
    color: var(--text-muted);
    font-weight: 500;
}

.flashcard-container {
    perspective: 1000px;
    margin-bottom: 2rem;
    cursor: pointer;
}

.flashcard {
    position: relative;
    width: 100%;
    height: 300px;
    transition: transform 0.6s cubic-bezier(0.4, 0, 0.2, 1);
    transform-style: preserve-3d;
}

.flashcard.flipped {
    transform: rotateY(180deg);
}

.flashcard-inner {
    position: relative;
    width: 100%;
    height: 100%;
    text-align: center;
}

.flashcard-front, .flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    padding: 2rem;
    border-radius: 1.5rem;
    background: var(--card-bg);
    box-shadow: var(--shadow);
    box-sizing: border-box;
}

.flashcard-back {
    transform: rotateY(180deg);
    background: #f1f5f9;
}

.label {
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 0.1em;
    color: var(--primary);
    font-weight: 700;
    margin-bottom: 1rem;
}

p {
    font-size: 1.5rem;
    line-height: 1.4;
    margin: 0;
    font-weight: 600;
}

.hint {
    margin-top: 2rem;
    font-size: 0.875rem;
    color: var(--text-muted);
    font-style: italic;
}

.controls {
    display: flex;
    justify-content: space-between;
    gap: 1rem;
}

.btn {
    background: var(--card-bg);
    border: 1px solid #e2e8f0;
    padding: 0.75rem 1.5rem;
    border-radius: 0.75rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
    flex: 1;
    color: var(--text-main);
}

.btn:hover:not(:disabled) {
    background: #f1f5f9;
    border-color: #cbd5e1;
    transform: translateY(-1px);
}

.btn:active:not(:disabled) {
    transform: translateY(0);
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}