:root {
    /* Modern Color Palette */
    --primary-color: #6f42c1; /* Indigo */
    --primary-hover: #5a32a3;
    --secondary-color: #0dcaf0; /* Cyan */
    --accent-color: #ffc107; /* Yellow/Gold */
    --success-color: #198754;
    --danger-color: #dc3545;
    
    /* Light Mode */
    --bg-color: #f3f4f6; /* Very light gray */
    --text-color: #1f2937; /* Gray 900 */
    --text-muted: #6b7280; /* Gray 500 */
    --card-bg: #ffffff;
    --card-border: rgba(0,0,0,0.05);
    --sidebar-bg: #ffffff;
    --sidebar-text: #374151;
    --sidebar-active-bg: #f3f4f6;
    --sidebar-active-text: #6f42c1;
    --input-bg: #ffffff;
    --input-border: #d1d5db;
    --input-text: #1f2937;
    
    /* Spacing & Radius */
    --border-radius: 12px;
    --card-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --transition-speed: 0.3s;
}

[data-theme="dark"] {
    /* Dark Mode */
    --bg-color: #111827; /* Gray 900 */
    --text-color: #f9fafb; /* Gray 50 */
    --text-muted: #9ca3af; /* Gray 400 */
    --card-bg: #1f2937; /* Gray 800 */
    --card-border: #374151;
    --sidebar-bg: #1f2937;
    --sidebar-text: #d1d5db;
    --sidebar-active-bg: #374151;
    --sidebar-active-text: #818cf8; /* Light Indigo */
    --input-bg: #374151;
    --input-border: #4b5563;
    --input-text: #f9fafb;
}

/* Global Styles */
body {
    background-color: var(--bg-color);
    color: var(--text-color);
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    transition: background-color var(--transition-speed), color var(--transition-speed);
    line-height: 1.6;
}

h1, h2, h3, h4, h5, h6 {
    color: var(--text-color);
    font-weight: 700;
    letter-spacing: -0.025em;
}

a {
    color: var(--primary-color);
    text-decoration: none;
    transition: color 0.2s;
}

a:hover {
    color: var(--primary-hover);
}

/* Cards */
.card {
    background-color: var(--card-bg);
    border: 1px solid var(--card-border);
    border-radius: var(--border-radius);
    box-shadow: var(--card-shadow);
    transition: transform 0.2s, box-shadow 0.2s;
    margin-bottom: 1.5rem;
}

.card:hover {
    transform: translateY(-2px);
    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
}

.card-header {
    background-color: transparent;
    border-bottom: 1px solid var(--card-border);
    padding: 1.25rem;
    font-weight: 600;
    color: var(--text-color);
}

.card-body {
    padding: 1.5rem;
    color: var(--text-color);
}

/* Buttons */
.btn {
    border-radius: 8px;
    padding: 0.5rem 1.25rem;
    font-weight: 500;
    transition: all 0.2s;
}

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

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

.btn-lg {
    padding: 0.75rem 1.5rem;
    font-size: 1.1rem;
}

/* Inputs */
.form-control, .form-select {
    background-color: var(--input-bg);
    border-color: var(--input-border);
    color: var(--input-text);
    border-radius: 8px;
    padding: 0.625rem 0.75rem;
}

.form-control:focus, .form-select:focus {
    background-color: var(--input-bg);
    color: var(--input-text);
    border-color: var(--primary-color);
    box-shadow: 0 0 0 0.25rem rgba(111, 66, 193, 0.25);
}

/* Sidebar (Admin/Professor) */
.sidebar {
    background-color: var(--sidebar-bg);
    border-right: 1px solid var(--card-border);
    height: 100vh;
    position: sticky;
    top: 0;
    padding-top: 1rem;
    overflow-y: auto;
}

.sidebar .nav-link {
    color: var(--sidebar-text);
    padding: 0.75rem 1.5rem;
    font-weight: 500;
    border-radius: 0 24px 24px 0;
    margin-right: 1rem;
    margin-bottom: 0.25rem;
    display: flex;
    align-items: center;
}

.sidebar .nav-link i {
    width: 24px;
    margin-right: 0.5rem;
    text-align: center;
}

.sidebar .nav-link:hover {
    background-color: rgba(0,0,0,0.03);
    color: var(--primary-color);
}

.sidebar .nav-link.active {
    background-color: var(--sidebar-active-bg);
    color: var(--sidebar-active-text);
    font-weight: 600;
}

/* Tables */
.table {
    color: var(--text-color);
    vertical-align: middle;
}

.table th {
    font-weight: 600;
    color: var(--text-muted);
    text-transform: uppercase;
    font-size: 0.75rem;
    letter-spacing: 0.05em;
    border-bottom-width: 1px;
}

[data-theme="dark"] .table {
    border-color: #374151;
}

[data-theme="dark"] .table-striped > tbody > tr:nth-of-type(odd) {
    --bs-table-accent-bg: rgba(255, 255, 255, 0.03);
    color: var(--text-color);
}

/* Navbar */
.navbar-custom {
    background-color: var(--card-bg);
    border-bottom: 1px solid var(--card-border);
    padding: 0.75rem 0;
}

.navbar-custom .navbar-brand {
    color: var(--primary-color);
    font-weight: 800;
    font-size: 1.5rem;
}

.navbar-custom .nav-link {
    color: var(--text-color);
    font-weight: 500;
}

.navbar-custom .nav-link:hover {
    color: var(--primary-color);
}

/* Utilities */
.text-muted {
    color: var(--text-muted) !important;
}

.badge {
    padding: 0.5em 0.75em;
    font-weight: 600;
    border-radius: 6px;
}

/* Progress Bars */
.progress {
    height: 0.75rem;
    border-radius: 1rem;
    background-color: rgba(0,0,0,0.1);
}

.progress-bar {
    border-radius: 1rem;
    background-color: var(--primary-color);
}

[data-theme="dark"] .progress {
    background-color: rgba(255,255,255,0.1);
}

/* Scrollbar Dark Mode */
[data-theme="dark"] ::-webkit-scrollbar {
    width: 10px;
}
[data-theme="dark"] ::-webkit-scrollbar-track {
    background: #121212; 
}
[data-theme="dark"] ::-webkit-scrollbar-thumb {
    background: #444; 
    border-radius: 5px;
}
[data-theme="dark"] ::-webkit-scrollbar-thumb:hover {
    background: #555; 
}

/* Course Player (Classroom) */
.video-container {
    background: #000;
    border-radius: var(--border-radius);
    overflow: hidden;
    box-shadow: var(--card-shadow);
}

.lesson-list .list-group-item {
    border: none;
    border-bottom: 1px solid var(--card-border);
    padding: 1rem;
    background-color: var(--card-bg);
    color: var(--text-color);
}

.lesson-list .list-group-item.active {
    background-color: var(--primary-color);
    color: white;
}

/* Home Page Custom Styles */
.hero-section {
    background: linear-gradient(135deg, #004e92 0%, #000428 100%);
    position: relative;
    overflow: hidden;
}

.hero-section::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-image: url('https://www.transparenttextures.com/patterns/cubes.png');
    opacity: 0.1;
}

.stats-section {
    background-color: #fff;
    border-bottom: 1px solid #e0e0e0;
}

.stat-card .icon-wrapper {
    transition: transform 0.3s ease;
}

.stat-card:hover .icon-wrapper {
    transform: scale(1.1);
}

.letter-spacing-1 {
    letter-spacing: 1px;
}

.hover-scale {
    transition: transform 0.3s ease;
}

.hover-scale:hover {
    transform: scale(1.05);
}

.hover-translate {
    transition: transform 0.3s ease;
}

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

.hover-shadow {
    transition: box-shadow 0.3s ease, transform 0.3s ease;
}

.hover-shadow:hover {
    box-shadow: 0 10px 20px rgba(0,0,0,0.1) !important;
    transform: translateY(-2px);
}

.transition-all {
    transition: all 0.3s ease;
}

@media (min-width: 768px) {
    .border-start-md {
        border-left: 1px solid #dee2e6;
    }
    .border-end-md {
        border-right: 1px solid #dee2e6;
    }
}