/* --- DESIGN SYSTEM & CSS VARIABLES --- */
:root {
    /* Color Palette (Light Mode - Default) */
    --color-primary: #0d6efd;
    --color-primary-light: #e7f1ff;
    --color-background: #f8f9fa;
    --color-surface: #ffffff; /* For cards, sidebars, headers */
    --color-text-primary: #212529;
    --color-text-secondary: #6c757d;
    --color-border: #dee2e6;

    /* Typography */
    --font-family-sans: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    --font-weight-normal: 400;
    --font-weight-medium: 500;
    --font-weight-semibold: 600;
    --font-weight-bold: 700;

    /* Spacing & Sizing */
    --sidebar-width: 250px;
    --sidebar-width-collapsed: 80px;
    --header-height: 70px;
    
    /* Borders & Shadows */
    --border-radius-sm: 0.25rem; /* 4px */
    --border-radius-md: 0.5rem;  /* 8px */
    --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.04);
    --shadow-md: 0 4px 8px rgba(0, 0, 0, 0.06);
    --shadow-lg: 0 10px 20px rgba(0, 0, 0, 0.08);

    /* Transitions */
    --transition-speed: 0.2s ease-in-out;
}

[data-theme="dark"] {
    --color-primary: #3b82f6;
    --color-primary-light: rgba(59, 130, 246, 0.1);
    --color-background: #121212;
    --color-surface: #1e1e1e;
    --color-text-primary: #f8f9fa;
    --color-text-secondary: #adb5bd;
    --color-border: #343a40;
}

/* --- GLOBAL & RESET STYLES --- */
*, *::before, *::after { box-sizing: border-box; }

body {
    margin: 0;
    font-family: var(--font-family-sans);
    font-weight: var(--font-weight-normal);
    background-color: var(--color-background);
    color: var(--color-text-primary);
    transition: background-color var(--transition-speed), color var(--transition-speed);
}

.site-wrapper { display: flex; }

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

/* --- SIDEBAR --- */
.sidebar {
    width: var(--sidebar-width);
    background-color: var(--color-surface);
    border-right: 1px solid var(--color-border);
    height: 100vh;
    position: fixed;
    top: 0; left: 0;
    display: flex;
    flex-direction: column;
    transition: width var(--transition-speed), background-color var(--transition-speed);
    overflow-y: auto;
}

.sidebar-header {
    display: flex;
    align-items: center;
    justify-content: center;
    height: var(--header-height);
    padding: 0 1.5rem;
    flex-shrink: 0;
}

.logo-full { display: block; }
.logo-icon { display: none; }

.sidebar-nav {
    flex-grow: 1;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    padding: 1rem 0;
}

.sidebar-nav ul { list-style: none; padding: 0; margin: 0; }
.sidebar-nav a.nav-link {
    display: flex;
    align-items: center;
    padding: 0.8rem 1.5rem; /* 12.8px 24px */
    margin: 0.25rem 1rem;
    color: var(--color-text-secondary);
    text-decoration: none;
    font-weight: var(--font-weight-medium);
    border-radius: var(--border-radius-md);
    transition: background-color var(--transition-speed), color var(--transition-speed);
}

.sidebar-nav a.nav-link:hover { color: var(--color-text-primary); background-color: var(--color-background); }
.sidebar-nav a.nav-link.active {
    color: var(--color-primary);
    background-color: var(--color-primary-light);
}

.sidebar-nav .fa-fw {
    width: 20px;
    margin-right: 1rem;
    text-align: center;
    font-size: 1.1em;
}

.sidebar-nav .nav-section-title {
    padding: 1.5rem 1.5rem 0.5rem;
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--color-text-secondary);
    font-weight: var(--font-weight-bold);
}

/* --- MAIN CONTENT & HEADER --- */
.main-content {
    margin-left: var(--sidebar-width);
    width: calc(100% - var(--sidebar-width));
    transition: margin-left var(--transition-speed), width var(--transition-speed);
}

.site-header {
    height: var(--header-height);
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0 2.5rem; /* 40px */
    background-color: var(--color-surface);
    border-bottom: 1px solid var(--color-border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.search-bar {
    display: flex;
    align-items: center;
    background-color: var(--color-background);
    border: 1px solid var(--color-border);
    border-radius: var(--border-radius-md);
    padding: 0.5rem 1rem; /* 8px 16px */
    width: 100%;
    max-width: 450px;
    transition: border-color var(--transition-speed), box-shadow var(--transition-speed);
}
.search-bar:focus-within {
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px var(--color-primary-light);
}
.search-bar i { color: var(--color-text-secondary); }
.search-bar input {
    border: none; outline: none; background: transparent;
    padding: 0 0.75rem; font-size: 1rem; width: 100%;
    color: var(--color-text-primary);
}

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

#theme-toggle {
    background: transparent;
    border: none;
    color: var(--color-text-secondary);
    font-size: 1.25rem;
    cursor: pointer;
    width: 40px; height: 40px;
    border-radius: 50%;
    display: grid;
    place-items: center;
    transition: color var(--transition-speed), background-color var(--transition-speed);
}
#theme-toggle:hover { color: var(--color-text-primary); background-color: var(--color-background); }
#theme-toggle .fa-sun { display: none; }
[data-theme="dark"] #theme-toggle .fa-sun { display: inline-block; }
[data-theme="dark"] #theme-toggle .fa-moon { display: none; }

#user-profile { display: flex; align-items: center; gap: 0.75rem; }
#user-avatar { width: 36px; height: 36px; border-radius: 50%; }

.btn {
    padding: 0.5rem 1rem;
    border-radius: var(--border-radius-sm);
    border: 1px solid transparent;
    font-weight: var(--font-weight-medium);
    cursor: pointer;
    transition: all var(--transition-speed);
}
.btn-secondary {
    background-color: var(--color-background);
    border-color: var(--color-border);
    color: var(--color-text-primary);
}
.btn-secondary:hover { background-color: var(--color-border); }

/* --- PAGE CONTENT & VIDEO GRID --- */
#app-content-wrapper { padding: 2.5rem; /* 40px */ }

.page-title {
    font-size: 2.25rem;
    font-weight: var(--font-weight-bold);
    margin-top: 0;
    margin-bottom: 2rem;
}

.video-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 2rem;
}

.video-card {
    background-color: var(--color-surface);
    border-radius: var(--border-radius-md);
    overflow: hidden;
    box-shadow: var(--shadow-md);
    text-decoration: none;
    color: var(--color-text-primary);
    transition: transform var(--transition-speed), box-shadow var(--transition-speed);
}
.video-card:hover { transform: translateY(-5px); box-shadow: var(--shadow-lg); }
.video-thumbnail {
    width: 100%;
    aspect-ratio: 16 / 9;
    object-fit: cover;
    background-color: var(--color-border);
    display: block;
    border-bottom: 1px solid var(--color-border);
}
.video-info { padding: 1rem; }
.video-title {
    font-size: 1.1rem;
    font-weight: var(--font-weight-semibold);
    margin: 0 0 0.25rem;
    display: -webkit-box;
    -webkit-line-clamp: 2;
    -webkit-box-orient: vertical;
    overflow: hidden;
}
.video-meta { font-size: 0.9rem; color: var(--color-text-secondary); }
.loading, .info-message {
    text-align: center;
    font-size: 1.2rem;
    padding: 4rem;
    color: var(--color-text-secondary);
}

/* --- RESPONSIVE DESIGN --- */
@media (max-width: 1024px) {
    .sidebar { width: var(--sidebar-width-collapsed); }
    .sidebar .nav-text, .sidebar .logo-full, .sidebar .nav-section-title .nav-text { display: none; }
    .sidebar .logo-icon { display: block; }
    .sidebar-nav a.nav-link { justify-content: center; }
    .sidebar-nav .fa-fw { margin-right: 0; }
    .main-content {
        width: calc(100% - var(--sidebar-width-collapsed));
        margin-left: var(--sidebar-width-collapsed);
    }
}

@media (max-width: 768px) {
    .site-header {
        height: auto;
        flex-direction: column;
        align-items: stretch;
        gap: 1rem;
        padding: 1rem;
    }
    .search-bar { max-width: 100%; }
    #app-content-wrapper { padding: 1.5rem; }
    .page-title { font-size: 1.75rem; }
}