:root {
    --primary: #6366f1;
    --primary-hover: #4f46e5;
    --secondary: #a855f7;
    --dark: #0f172a;
    --light: #f8fafc;
    --text-main: #1e293b;
    --text-muted: #64748b;
    --glass: rgba(255, 255, 255, 0.7);
    --glass-border: rgba(255, 255, 255, 0.3);
    --card-bg: #ffffff;
    --phrasal-color: #3b82f6;
    --connector-color: #10b981;
    --expression-color: #f59e0b;
    --vocabulary-color: #ec4899;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Outfit', 'Inter', sans-serif;
}

body {
    background: radial-gradient(circle at top left, #e0e7ff 0%, #f1f5f9 100%);
    color: var(--text-main);
    line-height: 1.6;
    min-height: 100vh;
}

header {
    padding: 3rem 1rem;
    text-align: center;
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    color: white;
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
    margin-bottom: 2rem;
}

header h1 {
    font-size: 2.5rem;
    font-weight: 800;
    margin-bottom: 0.5rem;
}

header p {
    font-size: 1.1rem;
    opacity: 0.9;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1rem;
}

.section-title {
    font-size: 1.8rem;
    font-weight: 700;
    margin: 2rem 0 1.5rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding-bottom: 0.5rem;
    border-bottom: 3px solid var(--primary);
    width: fit-content;
}

.phrasal-verbs-title {
    border-color: var(--phrasal-color);
}

.connectors-title {
    border-color: var(--connector-color);
}

.expressions-title {
    border-color: var(--expression-color);
}

.vocabulary-title {
    border-color: var(--vocabulary-color);
}

.grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 1.5rem;
    margin-bottom: 3rem;
}

/* Flashcard Styles */
.card-container {
    perspective: 1000px;
    height: 350px;
    cursor: pointer;
}

.card {
    position: relative;
    width: 100%;
    height: 100%;
    transform-style: preserve-3d;
    transition: transform 0.6s cubic-bezier(0.4, 0, 0.2, 1);
    border-radius: 1.5rem;
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.1);
}

.card.is-flipped {
    transform: rotateY(180deg);
}

.card-face {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    padding: 2rem;
    border-radius: 1.5rem;
    display: flex;
    flex-direction: column;
    justify-content: center;
    background: var(--card-bg);
    border: 1px solid var(--glass-border);
}

.card-front {
    background: linear-gradient(135deg, #ffffff 0%, #f8fafc 100%);
}

.card-back {
    transform: rotateY(180deg);
    background: linear-gradient(135deg, #ffffff 0%, #f1f5f9 100%);
    justify-content: flex-start;
    overflow-y: auto;
}

.card-front .type-badge {
    position: absolute;
    top: 1.5rem;
    right: 1.5rem;
    font-size: 0.75rem;
    font-weight: 700;
    text-transform: uppercase;
    padding: 0.25rem 0.75rem;
    border-radius: 9999px;
    background: #e2e8f0;
    color: #475569;
}

.card-front h3 {
    font-size: 1.25rem;
    color: var(--text-muted);
    font-weight: 500;
    margin-bottom: 1rem;
    font-style: italic;
}

.card-front p {
    font-size: 1.1rem;
    color: var(--text-main);
    font-weight: 600;
}

.card-back h2 {
    font-size: 1.5rem;
    color: var(--primary);
    margin-bottom: 0.5rem;
    border-bottom: 1px solid #e2e8f0;
    padding-bottom: 0.5rem;
}

.card-back .meaning {
    font-weight: 700;
    color: var(--dark);
    margin-bottom: 1rem;
    font-size: 1rem;
}

.card-back .example {
    background: #f1f5f9;
    padding: 1rem;
    border-radius: 0.75rem;
    margin-bottom: 1rem;
    border-left: 4px solid var(--primary);
    font-size: 0.95rem;
}

.card-back .example strong {
    color: var(--primary);
}

.card-back .usage {
    font-size: 0.85rem;
    color: var(--text-muted);
}

.card-back .usage b {
    color: #475569;
}

/* Decoration */
.click-hint {
    position: absolute;
    bottom: 1.5rem;
    left: 50%;
    transform: translateX(-50%);
    font-size: 0.75rem;
    color: var(--text-muted);
    display: flex;
    align-items: center;
    gap: 0.25rem;
}

/* Category Colors */
.card-phrasal .card-back h2 {
    color: var(--phrasal-color);
}

.card-phrasal .card-back .example {
    border-left-color: var(--phrasal-color);
}

.card-connector .card-back h2 {
    color: var(--connector-color);
}

.card-connector .card-back .example {
    border-left-color: var(--connector-color);
}

.card-expression .card-back h2 {
    color: var(--expression-color);
}

.card-expression .card-back .example {
    border-left-color: var(--expression-color);
}

.card-vocabulary .card-back h2 {
    color: var(--vocabulary-color);
}

.card-vocabulary .card-back .example {
    border-left-color: var(--vocabulary-color);
}

/* Navigation Styles */
.nav-section {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 2rem;
    padding: 2rem 0;
    margin-top: 2rem;
    border-top: 1px solid #e2e8f0;
}

.nav-btn {
    display: inline-flex;
    align-items: center;
    padding: 0.75rem 1.5rem;
    background: white;
    color: var(--primary);
    border: 2px solid var(--primary);
    border-radius: 9999px;
    text-decoration: none;
    font-weight: 700;
    transition: all 0.3s ease;
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
}

.nav-btn:hover {
    background: var(--primary);
    color: white;
    transform: translateY(-2px);
    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1);
}

.current-day-label {
    font-weight: 800;
    color: var(--text-muted);
    font-size: 1.1rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

footer {
    padding: 4rem 1rem;
    background: var(--dark);
    color: white;
    text-align: center;
    margin-top: 4rem;
}

.footer-links {
    display: flex;
    justify-content: center;
    gap: 2rem;
    margin-bottom: 1.5rem;
}

.footer-links a {
    color: #cbd5e1;
    text-decoration: none;
    font-size: 0.9rem;
}

.footer-links a:hover {
    color: white;
    text-decoration: underline;
}

.ads-space {
    width: 100%;
    min-height: 90px;
    margin: 2rem 0;
}

/* Day Picker Advanced Navigation */
#footer-day-picker {
    background: white;
    padding: 3rem 2rem;
    border-radius: 2rem;
    margin-top: 4rem;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.05);
}

#footer-day-picker h3 {
    text-align: center;
    margin-bottom: 2rem;
    font-size: 1.5rem;
    color: var(--dark);
}

.day-picker-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
    gap: 1rem;
}

.day-pick-item {
    display: flex;
    flex-direction: column;
    padding: 1rem;
    border: 2px solid #f1f5f9;
    border-radius: 1rem;
    text-decoration: none;
    transition: all 0.3s ease;
}

.day-pick-item:hover {
    border-color: var(--primary);
    background: #fdf2ff;
    transform: translateY(-3px);
}

.day-pick-item.active {
    background: var(--primary);
    border-color: var(--primary);
}

.day-pick-item .day-num {
    font-size: 0.8rem;
    font-weight: 800;
    color: var(--text-muted);
    text-transform: uppercase;
}

.day-pick-item.active .day-num,
.day-pick-item.active .day-title-short {
    color: white;
}

.day-pick-item .day-title-short {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--text-main);
    margin-top: 0.25rem;
}

@media (max-width: 640px) {
    header h1 {
        font-size: 1.8rem;
    }

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

    .card-container {
        height: 320px;
    }
}