:root {
    /* Color Palette */
    --bg-main: #000000; /* Pure Black */
    --bg-sidebar: #09090b; /* Zinc 950 */
    --bg-glass: rgba(9, 9, 11, 0.7);
    --border-glass: rgba(255, 255, 255, 0.08);
    
    --text-primary: #f8fafc; /* Slate 50 */
    --text-secondary: #94a3b8; /* Slate 400 */
    
    --accent-primary: #3b82f6; /* Blue 500 */
    --accent-primary-hover: #2563eb; /* Blue 600 */
    
    --color-again: #ef4444; /* Red 500 */
    --color-hard: #f59e0b; /* Amber 500 */
    --color-good: #10b981; /* Emerald 500 */
    --color-easy: #3b82f6; /* Blue 500 */
    
    --shadow-glass: 0 4px 30px rgba(0, 0, 0, 0.3);
    --border-radius: 16px;
    --border-radius-sm: 8px;
    
    --transition-fast: 0.2s ease;
    --transition-smooth: 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    background-color: var(--bg-main);
    color: var(--text-primary);
    min-height: 100vh;
    overflow-x: hidden;
}

/* Utilities */
.hidden {
    display: none !important;
}

.glass {
    background: var(--bg-glass);
    backdrop-filter: blur(12px);
    -webkit-backdrop-filter: blur(12px);
    border: 1px solid var(--border-glass);
    border-radius: var(--border-radius);
    box-shadow: var(--shadow-glass);
}

/* Layout */
.app-container {
    display: flex;
    height: 100vh;
}

/* Sidebar */
.sidebar {
    width: 260px;
    background-color: var(--bg-sidebar);
    padding: 2rem 1.5rem;
    display: flex;
    flex-direction: column;
    border-right: 1px solid var(--border-glass);
    z-index: 10;
}

.logo {
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--text-primary);
    display: flex;
    align-items: center;
    gap: 0.75rem;
    margin-bottom: 3rem;
}

.logo i {
    color: var(--accent-primary);
}

.logo img {
    width: 24px;
    height: 24px;
    object-fit: contain;
}

.nav-links {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.nav-links li {
    padding: 0.875rem 1rem;
    border-radius: var(--border-radius-sm);
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 1rem;
    color: var(--text-secondary);
    transition: var(--transition-fast);
    font-weight: 500;
}

.nav-links li:hover {
    background-color: rgba(255, 255, 255, 0.05);
    color: var(--text-primary);
}

.nav-links li.active {
    background-color: var(--accent-primary);
    color: white;
}

/* Main Content */
.main-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    overflow-y: auto;
    position: relative;
}

.top-header {
    padding: 2rem 3rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.top-header h1 {
    font-size: 2rem;
    font-weight: 700;
}

.view {
    padding: 0 3rem 3rem 3rem;
    animation: fadeIn var(--transition-smooth);
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

/* Buttons */
.btn {
    padding: 0.75rem 1.5rem;
    border: none;
    border-radius: var(--border-radius-sm);
    font-size: 0.95rem;
    font-weight: 600;
    font-family: inherit;
    cursor: pointer;
    transition: var(--transition-fast);
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
}

.btn-primary {
    background: linear-gradient(135deg, var(--accent-primary), #60a5fa);
    color: white;
    box-shadow: 0 4px 15px rgba(59, 130, 246, 0.3);
}

.btn-primary:hover {
    background: linear-gradient(135deg, var(--accent-primary-hover), var(--accent-primary));
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(59, 130, 246, 0.4);
}

.btn-secondary {
    background: rgba(255, 255, 255, 0.1);
    color: var(--text-primary);
    border: 1px solid var(--border-glass);
}

.btn-secondary:hover {
    background: rgba(255, 255, 255, 0.15);
}

.btn-icon {
    background: transparent;
    color: var(--text-secondary);
    padding: 0.5rem;
}

.btn-icon:hover {
    color: var(--text-primary);
}

.btn-large {
    padding: 1rem 2rem;
    font-size: 1.1rem;
}

.btn-close {
    background: transparent;
    border: none;
    color: var(--text-secondary);
    font-size: 1.25rem;
    cursor: pointer;
    transition: var(--transition-fast);
}

.btn-close:hover {
    color: var(--text-primary);
}

/* Dashboard Stats */
.stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 1.5rem;
    margin-bottom: 3rem;
}

.stat-card {
    padding: 1.5rem;
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    transition: transform var(--transition-fast);
}

.stat-card:hover {
    transform: translateY(-5px);
}

.stat-card h3 {
    font-size: 1rem;
    color: var(--text-secondary);
    font-weight: 500;
}

.stat-value {
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--text-primary);
}

/* Decks Grid */
.section-title {
    margin-bottom: 1.5rem;
    font-size: 1.25rem;
    color: var(--text-secondary);
}

.decks-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 1.5rem;
}

.deck-card {
    padding: 1.5rem;
    cursor: pointer;
    transition: transform var(--transition-smooth), box-shadow var(--transition-smooth);
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.deck-card:hover {
    transform: translateY(-5px) scale(1.02);
    box-shadow: 0 10px 40px rgba(0,0,0,0.4);
    border-color: rgba(255, 255, 255, 0.2);
}

.deck-title {
    font-size: 1.25rem;
    font-weight: 600;
}

.deck-desc {
    color: var(--text-secondary);
    font-size: 0.9rem;
    line-height: 1.5;
    flex: 1;
}

.deck-meta {
    display: flex;
    justify-content: space-between;
    font-size: 0.85rem;
    color: var(--text-secondary);
    border-top: 1px solid var(--border-glass);
    padding-top: 1rem;
}

.due-badge {
    background: rgba(239, 68, 68, 0.2);
    color: #fca5a5;
    padding: 0.25rem 0.5rem;
    border-radius: 4px;
    font-weight: 600;
}

/* Deck Detail View */
.deck-header {
    padding: 2rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 2rem;
}

.deck-actions {
    display: flex;
    gap: 1rem;
}

.cards-list-container {
    padding: 2rem;
}

.cards-list-container h3 {
    margin-bottom: 1.5rem;
    color: var(--text-secondary);
    font-size: 1.1rem;
}

.cards-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.card-list-item {
    padding: 1rem 1.5rem;
    background: rgba(255, 255, 255, 0.03);
    border: 1px solid var(--border-glass);
    border-radius: var(--border-radius-sm);
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: background var(--transition-fast);
}

.card-list-item:hover {
    background: rgba(255, 255, 255, 0.08);
}

.card-list-content {
    display: flex;
    gap: 2rem;
    flex: 1;
}

.card-list-front, .card-list-back {
    flex: 1;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.card-list-back {
    color: var(--text-secondary);
}

.card-list-actions {
    display: flex;
    gap: 0.5rem;
}

.card-list-actions button {
    background: none;
    border: none;
    color: var(--text-secondary);
    cursor: pointer;
    padding: 0.5rem;
    transition: color var(--transition-fast);
}

.card-list-actions button:hover {
    color: var(--text-primary);
}

/* Review View */
.review-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 2rem;
}

.progress-bar-container {
    flex: 1;
    height: 8px;
    background: rgba(255, 255, 255, 0.1);
    border-radius: 4px;
    margin: 0 2rem;
    overflow: hidden;
}

.progress-bar {
    height: 100%;
    background: var(--accent-primary);
    width: 0%;
    transition: width var(--transition-smooth);
}

.flashcard-container {
    perspective: 1000px;
    width: 100%;
    max-width: 800px;
    height: 400px;
    margin: 0 auto 3rem auto;
}

.flashcard {
    width: 100%;
    height: 100%;
    cursor: pointer;
}

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

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

.flashcard-front, .flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    -webkit-backface-visibility: hidden;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 3rem;
    font-size: 2rem;
    text-align: center;
    line-height: 1.4;
    transition: opacity 0s 0.25s; /* Hide halfway through the flip */
}

.flashcard-front {
    opacity: 1;
}

.flashcard.flipped .flashcard-front {
    opacity: 0;
}

.flashcard-back {
    transform: rotateX(180deg);
    background: rgba(59, 130, 246, 0.1); /* Slight tint for the back */
    border-color: rgba(59, 130, 246, 0.3);
    opacity: 0;
}

.flashcard.flipped .flashcard-back {
    opacity: 1;
}

.review-actions {
    display: flex;
    justify-content: center;
    min-height: 80px;
}

.srs-actions {
    display: flex;
    gap: 1rem;
    width: 100%;
    max-width: 800px;
}

.btn-srs {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 1rem;
    border-radius: var(--border-radius-sm);
    border: none;
    cursor: pointer;
    transition: transform var(--transition-fast), filter var(--transition-fast);
    color: white;
}

.btn-srs:hover {
    transform: translateY(-3px);
    filter: brightness(1.1);
}

.btn-again { background-color: var(--color-again); }
.btn-hard { background-color: var(--color-hard); }
.btn-good { background-color: var(--color-good); }
.btn-easy { background-color: var(--color-easy); }

.grade-label {
    font-weight: 700;
    font-size: 1.1rem;
    margin-bottom: 0.25rem;
}

.grade-time {
    font-size: 0.85rem;
    opacity: 0.9;
}

/* Modals */
.modal-overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.6);
    backdrop-filter: blur(4px);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 100;
    opacity: 1;
    transition: opacity var(--transition-fast);
}

.modal-overlay.hidden {
    opacity: 0;
    pointer-events: none;
}

.modal {
    width: 100%;
    max-width: 500px;
    transform: scale(1);
    transition: transform var(--transition-smooth);
    padding: 2rem;
}

.modal-overlay.hidden .modal {
    transform: scale(0.95);
}

.modal-large {
    max-width: 700px;
}

.modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1.5rem;
}

.form-group {
    margin-bottom: 1.5rem;
}

.form-group label {
    display: block;
    margin-bottom: 0.5rem;
    color: var(--text-secondary);
    font-weight: 500;
}

.form-group input, .form-group textarea {
    width: 100%;
    padding: 0.875rem 1rem;
    border-radius: var(--border-radius-sm);
    border: 1px solid var(--border-glass);
    background: rgba(0, 0, 0, 0.2);
    color: var(--text-primary);
    font-family: inherit;
    font-size: 1rem;
    transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.form-group input:focus, .form-group textarea:focus {
    outline: none;
    border-color: var(--accent-primary);
    box-shadow: 0 0 0 2px rgba(59, 130, 246, 0.2);
}

.form-group textarea {
    resize: vertical;
    min-height: 100px;
}

.form-group textarea.textarea-large {
    min-height: 150px;
}

.modal-footer {
    display: flex;
    justify-content: flex-end;
    gap: 1rem;
    margin-top: 2rem;
}

/* Mobile Responsive Styles */
.mobile-menu-btn {
    display: none;
    font-size: 1.5rem;
    color: var(--text-primary);
}

.header-left {
    display: flex;
    align-items: center;
    gap: 1rem;
}

.sidebar-overlay {
    position: fixed;
    inset: 0;
    background: rgba(0,0,0,0.5);
    backdrop-filter: blur(2px);
    z-index: 999;
    display: none;
    opacity: 0;
    transition: opacity var(--transition-fast);
}

.sidebar-overlay.active {
    display: block;
    opacity: 1;
}

@media (max-width: 768px) {
    .app-container {
        flex-direction: column;
    }

    .sidebar {
        position: fixed;
        left: -100%;
        top: 0;
        bottom: 0;
        width: 280px;
        z-index: 1000;
        transition: left var(--transition-smooth);
        background: var(--bg-sidebar);
    }

    .sidebar.open {
        left: 0;
        box-shadow: 0 0 50px rgba(0,0,0,0.8);
    }

    .mobile-menu-btn {
        display: block;
    }

    .hide-mobile {
        display: none;
    }

    .top-header {
        padding: 1rem 1.5rem;
    }

    .view {
        padding: 0 1.5rem 1.5rem 1.5rem;
    }

    .stats-grid {
        grid-template-columns: 1fr;
    }
    
    .decks-grid {
        grid-template-columns: 1fr;
    }
    
    .card-list-content {
        flex-direction: column;
        gap: 0.5rem;
    }
    
    .flashcard-front, .flashcard-back {
        padding: 1.5rem;
        font-size: 1.25rem;
    }
    
    .srs-actions {
        flex-wrap: wrap;
    }
    
    .btn-srs {
        flex-basis: 45%;
        padding: 0.75rem;
    }
    
    .deck-header {
        flex-direction: column;
        align-items: flex-start;
        gap: 1rem;
        padding: 1.5rem;
    }
    
    .deck-actions {
        width: 100%;
        justify-content: space-between;
    }
    
    .review-header {
        flex-wrap: wrap;
        gap: 1rem;
    }
    
    .progress-bar-container {
        order: 3;
        width: 100%;
        margin: 0;
    }
}