:root {
    --bg-dark: #0f172a;
    --bg-darker: #020617;
    --primary: #10b981; /* Emerald */
    --primary-hover: #059669;
    --text-primary: #f8fafc;
    --text-secondary: #94a3b8;
    --glass-bg: rgba(30, 41, 59, 0.7);
    --glass-border: rgba(255, 255, 255, 0.1);
    --card-bg: #1e293b;
    --danger: #ef4444;
    --success: #10b981;
    --warning: #eab308;
    
    --radius-sm: 6px;
    --radius-md: 12px;
    --radius-lg: 16px;
    
    --transition: 0.3s ease;
}

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

body {
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
    background-color: var(--bg-darker);
    background-image: 
        radial-gradient(at 0% 0%, rgba(16, 185, 129, 0.1) 0px, transparent 50%),
        radial-gradient(at 100% 100%, rgba(30, 41, 59, 0.5) 0px, transparent 50%);
    color: var(--text-primary);
    min-height: 100vh;
    display: flex;
    overflow: hidden;
}

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

.hidden {
    display: none !important;
}

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

.sidebar {
    width: 280px;
    padding: 24px;
    display: flex;
    flex-direction: column;
    gap: 32px;
    border-right: 1px solid var(--glass-border);
    overflow-y: auto;
}

.main-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    overflow: hidden;
}

.app-title {
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary);
    letter-spacing: -0.5px;
}

/* Sidebar Sections */
.progress-section h3, .timer-section h3, .categories-nav h3 {
    font-size: 0.875rem;
    text-transform: uppercase;
    letter-spacing: 1px;
    color: var(--text-secondary);
    margin-bottom: 12px;
}

.progress-bar-container {
    height: 8px;
    background: rgba(0, 0, 0, 0.3);
    border-radius: var(--radius-sm);
    overflow: hidden;
    margin-bottom: 8px;
}

.progress-bar {
    height: 100%;
    background: var(--primary);
    width: 0%;
    transition: width 0.5s ease;
}

.progress-stats {
    font-size: 0.875rem;
    color: var(--text-secondary);
}

.timer-display {
    font-size: 2rem;
    font-variant-numeric: tabular-nums;
    font-weight: 600;
    margin-bottom: 12px;
}

.timer-controls {
    display: flex;
    gap: 8px;
}

.categories-nav ul {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.category-item {
    padding: 8px 12px;
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    justify-content: space-between;
    align-items: center;
    color: var(--text-secondary);
}

.category-item:hover {
    background: rgba(255, 255, 255, 0.05);
    color: var(--text-primary);
}

.category-item.active {
    background: rgba(16, 185, 129, 0.1);
    color: var(--primary);
    font-weight: 500;
}

.category-count-badge {
    background: rgba(255, 255, 255, 0.1);
    padding: 2px 6px;
    border-radius: 12px;
    font-size: 0.75rem;
}

/* Header */
.topbar {
    height: 70px;
    padding: 0 32px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-bottom: 1px solid var(--glass-border);
}

.search-input {
    background: rgba(0, 0, 0, 0.2);
    border: 1px solid var(--glass-border);
    color: var(--text-primary);
    padding: 10px 16px;
    border-radius: var(--radius-md);
    width: 300px;
    font-family: inherit;
    outline: none;
    transition: var(--transition);
}

.search-input:focus {
    border-color: var(--primary);
    box-shadow: 0 0 0 2px rgba(16, 185, 129, 0.2);
}

.current-category {
    display: flex;
    align-items: center;
    gap: 12px;
    font-weight: 600;
}

.badge {
    background: var(--primary);
    color: white;
    padding: 4px 8px;
    border-radius: var(--radius-sm);
    font-size: 0.875rem;
}

/* Study Area */
.study-area {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 40px;
    overflow-y: auto;
}

.flashcard-container {
    width: 100%;
    max-width: 600px;
    height: 400px;
    perspective: 1000px;
    margin-bottom: 32px;
}

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

.flashcard-inner {
    position: relative;
    width: 100%;
    height: 100%;
    text-align: center;
    transition: transform 0.6s cubic-bezier(0.4, 0.0, 0.2, 1);
    transform-style: preserve-3d;
}

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

.flashcard-front, .flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    -webkit-backface-visibility: hidden;
    backface-visibility: hidden;
    background: var(--card-bg);
    border: 1px solid var(--glass-border);
    border-radius: var(--radius-md);
    padding: 40px;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.2);
}

.flashcard-back {
    transform: rotateY(180deg);
    background: rgba(30, 41, 59, 0.9);
}

.card-category-badge {
    position: absolute;
    top: 20px;
    left: 20px;
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 1px;
    color: var(--primary);
    background: rgba(16, 185, 129, 0.1);
    padding: 4px 10px;
    border-radius: var(--radius-sm);
}

.card-question {
    font-size: 1.75rem;
    font-weight: 600;
    line-height: 1.4;
    margin-bottom: 24px;
}

.card-hint {
    position: absolute;
    bottom: 20px;
    color: var(--text-secondary);
    font-size: 0.875rem;
}

.card-answer {
    font-size: 1.125rem;
    line-height: 1.6;
    color: var(--text-primary);
    text-align: left;
    width: 100%;
    overflow-y: auto;
    max-height: 100%;
    white-space: pre-wrap;
    padding-right: 10px;
}

/* Controls */
.card-controls {
    display: flex;
    gap: 16px;
    margin-bottom: 24px;
}

.btn {
    background: rgba(255, 255, 255, 0.1);
    border: 1px solid var(--glass-border);
    color: var(--text-primary);
    padding: 12px 24px;
    border-radius: var(--radius-md);
    cursor: pointer;
    font-family: inherit;
    font-weight: 500;
    font-size: 1rem;
    transition: var(--transition);
}

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

.btn-primary {
    background: var(--primary);
    border-color: var(--primary);
    color: #fff;
}

.btn-primary:hover {
    background: var(--primary-hover);
}

.btn-small {
    padding: 8px 16px;
    font-size: 0.875rem;
}

.btn-action {
    background: rgba(16, 185, 129, 0.2);
    border-color: var(--primary);
    color: var(--primary);
}

.btn-action:hover {
    background: rgba(16, 185, 129, 0.3);
}

.rating-controls {
    text-align: center;
    background: var(--glass-bg);
    padding: 20px;
    border-radius: var(--radius-md);
    border: 1px solid var(--glass-border);
    animation: slideUp 0.3s ease;
}

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

.rating-controls p {
    margin-bottom: 16px;
    color: var(--text-secondary);
}

.rating-buttons {
    display: flex;
    gap: 8px;
}

.btn-rating {
    padding: 10px 16px;
    font-size: 0.875rem;
}

.btn-rating[data-rating="1"], .btn-rating[data-rating="2"] { color: var(--success); border-color: rgba(16, 185, 129, 0.3); }
.btn-rating[data-rating="3"] { color: var(--warning); border-color: rgba(234, 179, 8, 0.3); }
.btn-rating[data-rating="4"], .btn-rating[data-rating="5"] { color: var(--danger); border-color: rgba(239, 68, 68, 0.3); }

/* Modal */
.modal {
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    background: rgba(0, 0, 0, 0.7);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    padding: 20px;
}

.modal-content {
    width: 100%;
    max-width: 500px;
    padding: 32px;
    border-radius: var(--radius-lg);
}

.modal-content h2 {
    margin-bottom: 24px;
}

.form-group {
    margin-bottom: 20px;
}

.form-group label {
    display: block;
    margin-bottom: 8px;
    color: var(--text-secondary);
    font-size: 0.875rem;
}

.form-group input, .form-group textarea {
    width: 100%;
    background: rgba(0, 0, 0, 0.2);
    border: 1px solid var(--glass-border);
    color: var(--text-primary);
    padding: 12px;
    border-radius: var(--radius-sm);
    font-family: inherit;
    resize: vertical;
}

.modal-actions {
    display: flex;
    justify-content: flex-end;
    gap: 12px;
    margin-top: 32px;
}

.empty-state {
    text-align: center;
    color: var(--text-secondary);
    margin-top: 100px;
}