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

:root {
    --deep-blue: #0F172A;
    --midnight-blue: #1E293B;
    --slate-blue: #334155;
    --primary-blue: #3B82F6;
    --bright-blue: #60A5FA;
    --sky-blue: #7DD3FC;
    --light-blue: #BAE6FD;
    --ice-blue: #E0F2FE;
    --pale-blue: #F0F9FF;

    --bg-gradient-start: #0A1929;
    --bg-gradient-end: #1A365D;

    --card-bg: #FFFFFF;
    --card-bg-alt: #F8FAFC;

    --text-primary: #0F172A;
    --text-secondary: #475569;
    --text-muted: #94A3B8;
    --text-white: #FFFFFF;

    --border-light: #E2E8F0;
    --border-medium: #CBD5E1;

    --success-color: #10B981;
    --error-color: #EF4444;

    --shadow-sm: 0 1px 2px 0 rgba(15, 23, 42, 0.05);
    --shadow: 0 4px 6px -1px rgba(15, 23, 42, 0.08), 0 2px 4px -2px rgba(15, 23, 42, 0.04);
    --shadow-md: 0 10px 15px -3px rgba(15, 23, 42, 0.1), 0 4px 6px -4px rgba(15, 23, 42, 0.05);
    --shadow-lg: 0 20px 25px -5px rgba(15, 23, 42, 0.12), 0 8px 10px -6px rgba(15, 23, 42, 0.08);
    --shadow-xl: 0 25px 50px -12px rgba(15, 23, 42, 0.25);

    --shadow-blue: 0 10px 40px -10px rgba(59, 130, 246, 0.3);
    --shadow-blue-lg: 0 20px 60px -15px rgba(59, 130, 246, 0.4);
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    background: linear-gradient(135deg, var(--bg-gradient-start) 0%, var(--bg-gradient-end) 100%);
    background-attachment: fixed;
    min-height: 100vh;
    padding: 30px 20px;
    color: var(--text-primary);
    position: relative;
    overflow-x: hidden;
}

body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-image:
        radial-gradient(circle at 20% 30%, rgba(59, 130, 246, 0.1) 0%, transparent 50%),
        radial-gradient(circle at 80% 70%, rgba(96, 165, 250, 0.1) 0%, transparent 50%);
    pointer-events: none;
    z-index: 0;
}

.container {
    max-width: 1280px;
    margin: 0 auto;
    background: var(--card-bg);
    border-radius: 24px;
    padding: 40px;
    box-shadow: var(--shadow-xl);
    position: relative;
    z-index: 1;
    border: 1px solid rgba(255, 255, 255, 0.1);
}

header {
    text-align: center;
    margin-bottom: 50px;
    padding-bottom: 30px;
    border-bottom: 1px solid var(--border-light);
}

h1 {
    font-size: 3rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--primary-blue) 0%, var(--sky-blue) 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    margin-bottom: 12px;
    letter-spacing: -0.02em;
}

.subtitle {
    color: var(--text-secondary);
    font-size: 1.125rem;
    font-weight: 400;
    letter-spacing: -0.01em;
}

.stats {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
    gap: 24px;
    margin-bottom: 40px;
}

.stat-card {
    background: linear-gradient(135deg, var(--primary-blue) 0%, var(--bright-blue) 100%);
    padding: 28px 24px;
    border-radius: 16px;
    box-shadow: var(--shadow-blue);
    text-align: center;
    border: none;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    position: relative;
    overflow: hidden;
}

.stat-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(135deg, rgba(255, 255, 255, 0.1) 0%, transparent 100%);
    opacity: 0;
    transition: opacity 0.3s;
}

.stat-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-blue-lg);
}

.stat-card:hover::before {
    opacity: 1;
}

.stat-number {
    display: block;
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--text-white);
    margin-bottom: 6px;
    position: relative;
}

.stat-label {
    color: rgba(255, 255, 255, 0.9);
    font-size: 0.875rem;
    text-transform: uppercase;
    letter-spacing: 1px;
    font-weight: 500;
    position: relative;
}

.controls {
    margin-bottom: 40px;
}

.tabs {
    display: flex;
    gap: 12px;
    margin-bottom: 24px;
    flex-wrap: wrap;
    padding: 6px;
    background: var(--card-bg-alt);
    border-radius: 12px;
    border: 1px solid var(--border-light);
}

.tab-btn {
    padding: 12px 28px;
    background: transparent;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--text-secondary);
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    position: relative;
}

.tab-btn:hover {
    color: var(--primary-blue);
    background: rgba(59, 130, 246, 0.08);
}

.tab-btn.active {
    background: linear-gradient(135deg, var(--primary-blue), var(--bright-blue));
    color: var(--text-white);
    box-shadow: var(--shadow-blue);
}

.search-filter {
    display: flex;
    gap: 16px;
    flex-wrap: wrap;
}

#searchInput,
#categoryFilter {
    flex: 1;
    min-width: 220px;
    padding: 14px 18px;
    border: 2px solid var(--border-light);
    border-radius: 12px;
    font-size: 0.9375rem;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    background: var(--card-bg);
    color: var(--text-primary);
}

#searchInput:focus,
#categoryFilter:focus {
    outline: none;
    border-color: var(--primary-blue);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

.content {
    min-height: 400px;
}

.tab-content {
    display: none;
}

.tab-content.active {
    display: block;
    animation: fadeInUp 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

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

.expressions-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
    gap: 24px;
}

.expression-card {
    background: var(--card-bg);
    padding: 24px;
    border-radius: 16px;
    box-shadow: var(--shadow);
    border: 1px solid var(--border-light);
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    cursor: pointer;
    position: relative;
    overflow: hidden;
}

.expression-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 4px;
    background: linear-gradient(90deg, var(--primary-blue), var(--sky-blue));
    transform: scaleX(0);
    transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.expression-card:hover {
    transform: translateY(-6px);
    box-shadow: var(--shadow-lg);
    border-color: var(--primary-blue);
}

.expression-card:hover::before {
    transform: scaleX(1);
}

.expression-card.learned {
    background: linear-gradient(135deg, rgba(16, 185, 129, 0.05), rgba(16, 185, 129, 0.02));
    border-color: var(--success-color);
}

.expression-card.learned::before {
    background: var(--success-color);
    transform: scaleX(1);
}

.expression-title {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--primary-blue);
    margin-bottom: 12px;
    letter-spacing: -0.01em;
}

.expression-category {
    display: inline-block;
    padding: 6px 14px;
    background: linear-gradient(135deg, var(--primary-blue), var(--bright-blue));
    color: white;
    border-radius: 20px;
    font-size: 0.75rem;
    margin-bottom: 14px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    font-weight: 600;
    box-shadow: var(--shadow-sm);
}

.expression-meaning {
    color: var(--text-primary);
    margin-bottom: 12px;
    line-height: 1.7;
    font-size: 0.9375rem;
}

.expression-example {
    color: var(--text-secondary);
    font-style: italic;
    font-size: 0.875rem;
    padding: 12px 16px;
    background: var(--pale-blue);
    border-left: 3px solid var(--primary-blue);
    border-radius: 0 8px 8px 0;
    line-height: 1.6;
}

.mark-learned {
    margin-top: 16px;
    padding: 10px 20px;
    background: linear-gradient(135deg, var(--success-color), #059669);
    color: white;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.875rem;
    font-weight: 500;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    box-shadow: var(--shadow-sm);
}

.mark-learned:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px -5px rgba(16, 185, 129, 0.4);
}

.mark-learned.learned {
    background: var(--text-muted);
}

.categories-container {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
    gap: 24px;
}

.category-card {
    background: var(--card-bg);
    padding: 36px 24px;
    border-radius: 16px;
    box-shadow: var(--shadow);
    border: 1px solid var(--border-light);
    text-align: center;
    cursor: pointer;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    position: relative;
    overflow: hidden;
}

.category-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(135deg, rgba(59, 130, 246, 0.05), rgba(125, 211, 252, 0.05));
    opacity: 0;
    transition: opacity 0.3s;
}

.category-card:hover {
    transform: translateY(-6px);
    box-shadow: var(--shadow-lg);
    border-color: var(--primary-blue);
}

.category-card:hover::before {
    opacity: 1;
}

.category-icon {
    font-size: 3rem;
    margin-bottom: 16px;
    position: relative;
}

.category-name {
    font-size: 1.25rem;
    font-weight: 600;
    margin-bottom: 8px;
    color: var(--text-primary);
    position: relative;
}

.category-count {
    color: var(--text-secondary);
    font-size: 0.875rem;
    position: relative;
}

.flashcard-container {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 32px;
}

.flashcard {
    width: 100%;
    max-width: 650px;
    height: 420px;
    perspective: 1500px;
    cursor: pointer;
}

.flashcard-inner {
    position: relative;
    width: 100%;
    height: 100%;
    transition: transform 0.7s cubic-bezier(0.4, 0, 0.2, 1);
    transform-style: preserve-3d;
}

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

.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: 48px;
    background: var(--card-bg);
    border-radius: 20px;
    box-shadow: var(--shadow-lg);
    border: 1px solid var(--border-light);
}

.flashcard-front {
    background: var(--card-bg);
}

.flashcard-front h2 {
    font-size: 2.25rem;
    font-weight: 600;
    background: linear-gradient(135deg, var(--primary-blue), var(--sky-blue));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    text-align: center;
    margin-bottom: 24px;
    letter-spacing: -0.02em;
}

.flashcard-front .hint {
    color: var(--text-muted);
    font-size: 0.9375rem;
    padding: 10px 20px;
    background: var(--pale-blue);
    border-radius: 20px;
}

.flashcard-back {
    transform: rotateY(180deg);
    background: linear-gradient(135deg, var(--primary-blue) 0%, var(--bright-blue) 100%);
    color: white;
    border: none;
}

.flashcard-back p {
    font-size: 1.25rem;
    line-height: 1.8;
    text-align: center;
    margin-bottom: 20px;
    font-weight: 400;
}

.flashcard-controls {
    display: flex;
    gap: 24px;
    align-items: center;
}

.btn-primary,
.btn-secondary {
    padding: 14px 32px;
    border: none;
    border-radius: 12px;
    font-size: 0.9375rem;
    cursor: pointer;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    font-weight: 500;
}

.btn-primary {
    background: linear-gradient(135deg, var(--primary-blue), var(--bright-blue));
    color: white;
    box-shadow: var(--shadow-blue);
}

.btn-primary:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-blue-lg);
}

.btn-secondary {
    background: var(--card-bg);
    color: var(--text-primary);
    border: 2px solid var(--border-medium);
}

.btn-secondary:hover {
    border-color: var(--primary-blue);
    color: var(--primary-blue);
    background: rgba(59, 130, 246, 0.05);
}

#cardCounter {
    font-weight: 600;
    color: var(--text-primary);
    font-size: 1.125rem;
}

.quiz-container {
    max-width: 750px;
    margin: 0 auto;
}

.quiz-start,
.quiz-results {
    text-align: center;
    padding: 48px;
    background: var(--card-bg);
    border-radius: 20px;
    box-shadow: var(--shadow-lg);
    border: 1px solid var(--border-light);
}

.quiz-start h2,
.quiz-results h2 {
    font-size: 2.25rem;
    margin-bottom: 16px;
    background: linear-gradient(135deg, var(--primary-blue), var(--sky-blue));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    font-weight: 700;
}

.quiz-start p {
    color: var(--text-secondary);
    margin-bottom: 36px;
    font-size: 1.125rem;
    line-height: 1.7;
}

.quiz-question {
    background: var(--card-bg);
    padding: 36px;
    border-radius: 20px;
    box-shadow: var(--shadow-lg);
    border: 1px solid var(--border-light);
}

.quiz-header {
    display: flex;
    justify-content: space-between;
    margin-bottom: 36px;
    font-weight: 600;
    color: var(--text-secondary);
    font-size: 0.9375rem;
}

#questionExpression {
    font-size: 2rem;
    background: linear-gradient(135deg, var(--primary-blue), var(--sky-blue));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    text-align: center;
    margin-bottom: 36px;
    font-weight: 600;
}

.quiz-options {
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.quiz-option {
    padding: 20px 24px;
    background: var(--card-bg);
    border: 2px solid var(--border-light);
    border-radius: 12px;
    cursor: pointer;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
    text-align: left;
    font-size: 1rem;
    position: relative;
    overflow: hidden;
}

.quiz-option::before {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    bottom: 0;
    width: 4px;
    background: var(--primary-blue);
    transform: scaleY(0);
    transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.quiz-option:hover {
    border-color: var(--primary-blue);
    background: var(--pale-blue);
}

.quiz-option:hover::before {
    transform: scaleY(1);
}

.quiz-option.correct {
    border-color: var(--success-color);
    background: rgba(16, 185, 129, 0.1);
}

.quiz-option.correct::before {
    background: var(--success-color);
    transform: scaleY(1);
}

.quiz-option.incorrect {
    border-color: var(--error-color);
    background: rgba(239, 68, 68, 0.1);
}

.quiz-option.incorrect::before {
    background: var(--error-color);
    transform: scaleY(1);
}

.quiz-option.disabled {
    cursor: not-allowed;
    opacity: 0.7;
}

.final-score {
    font-size: 4rem;
    background: linear-gradient(135deg, var(--primary-blue), var(--sky-blue));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    margin: 36px 0;
    font-weight: 700;
}

.hidden {
    display: none;
}

@media (max-width: 768px) {
    body {
        padding: 20px 15px;
    }

    h1 {
        font-size: 2rem;
    }

    .container {
        padding: 24px;
        border-radius: 16px;
    }

    .stats {
        grid-template-columns: 1fr;
    }

    .expressions-grid,
    .categories-container {
        grid-template-columns: 1fr;
    }

    .flashcard {
        height: 350px;
    }

    .flashcard-front h2 {
        font-size: 1.75rem;
    }

    .tabs {
        flex-direction: column;
        gap: 8px;
    }

    .tab-btn {
        width: 100%;
    }

    .quiz-start,
    .quiz-results {
        padding: 32px 24px;
    }

    .final-score {
        font-size: 3rem;
    }
}

@media (min-width: 1024px) {
    .expressions-grid {
        grid-template-columns: repeat(auto-fill, minmax(360px, 1fr));
    }
}