/* ============================================
   STEM Hub - Main Stylesheet
   Modern, Clean Design inspired by MDN, Stripe Docs, Notion
   ============================================ */

/* --- CSS Variables (Theme Configuration) --- */
:root {
    /* Colors - Dark Theme */
    --bg-primary: #0d1117;
    --bg-secondary: #161b22;
    --bg-tertiary: #21262d;
    --bg-hover: #30363d;
    --bg-active: #388bfd26;
    
    --text-primary: #e6edf3;
    --text-secondary: #8b949e;
    --text-muted: #6e7681;
    
    --accent-primary: #58a6ff;
    --accent-secondary: #1f6feb;
    --accent-hover: #79c0ff;
    --accent-glow: rgba(88, 166, 255, 0.15);
    
    --success: #3fb950;
    --warning: #d29922;
    --error: #f85149;
    
    --border-color: #30363d;
    --border-subtle: #21262d;
    --shadow-color: rgba(0, 0, 0, 0.3);
    --shadow-lg: 0 8px 24px rgba(0, 0, 0, 0.4);
    
    /* Spacing */
    --sidebar-width: 280px;
    --sidebar-collapsed-width: 64px;
    --content-padding: 48px;
    --content-max-width: 900px;
    --border-radius: 8px;
    --border-radius-lg: 12px;
    
    /* Typography */
    --font-sans: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Noto Sans', Helvetica, Arial, sans-serif;
    --font-mono: 'SFMono-Regular', Consolas, 'Liberation Mono', Menlo, monospace;
    
    /* Transitions */
    --transition-fast: 0.15s ease;
    --transition-normal: 0.25s ease;
    --transition-slow: 0.35s ease;
}

/* --- Global Reset & Base Styles --- */
*, *::before, *::after {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    font-family: var(--font-sans);
    font-size: 16px;
    margin: 0;
    padding: 0;
    display: flex;
    background-color: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.7;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

::selection {
    background-color: var(--accent-primary);
    color: var(--bg-primary);
}

/* --- Scrollbar Styling --- */
::-webkit-scrollbar {
    width: 8px;
    height: 8px;
}

::-webkit-scrollbar-track {
    background: var(--bg-secondary);
}

::-webkit-scrollbar-thumb {
    background: var(--border-color);
    border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
    background: var(--text-muted);
}

/* --- Sidebar Styles --- */
.sidebar {
    width: var(--sidebar-width);
    background-color: var(--bg-secondary);
    border-right: 1px solid var(--border-color);
    padding: 16px 0;
    transition: width var(--transition-normal), transform var(--transition-normal);
    height: 100vh;
    position: fixed;
    overflow-y: auto;
    overflow-x: hidden;
    z-index: 100;
}

.sidebar.collapsed {
    width: var(--sidebar-collapsed-width);
}

.sidebar.collapsed .topic-header,
.sidebar.collapsed .subtopic-list,
.sidebar.collapsed h3,
.sidebar.collapsed .nav-link {
    display: none;
}

/* --- Sidebar Header --- */
.sidebar-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 8px 16px 20px 20px;
    border-bottom: 1px solid var(--border-color);
    margin-bottom: 12px;
}

.sidebar-header h3 {
    margin: 0;
    font-size: 1.25rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--accent-primary), #a371f7);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    letter-spacing: -0.02em;
}

.toggle-btn {
    background: transparent;
    border: 1px solid var(--border-color);
    color: var(--text-secondary);
    font-size: 1.1rem;
    cursor: pointer;
    padding: 6px 10px;
    border-radius: var(--border-radius);
    transition: all var(--transition-fast);
    display: flex;
    align-items: center;
    justify-content: center;
}

.toggle-btn:hover {
    background-color: var(--bg-hover);
    color: var(--text-primary);
    border-color: var(--text-muted);
}

/* --- Home Link Styles --- */
.nav-link {
    padding: 8px 12px;
    margin-bottom: 8px;
}

.home-link {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 10px 16px;
    font-weight: 600;
    color: var(--text-primary);
    text-decoration: none;
    font-size: 0.95rem;
    border-radius: var(--border-radius);
    transition: all var(--transition-fast);
    margin: 0 8px;
}

.home-link:hover {
    background-color: var(--bg-hover);
}

.home-link.active-link {
    background-color: var(--bg-active);
    color: var(--accent-primary);
}

/* --- Topic & Subtopic Styles --- */
.topic {
    margin-bottom: 4px;
}

.topic-header {
    padding: 10px 20px;
    font-size: 0.8rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    cursor: pointer;
    color: var(--text-secondary);
    margin: 0;
    transition: all var(--transition-fast);
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.topic-header:hover {
    color: var(--text-primary);
    background-color: var(--bg-hover);
}

.topic-header::after {
    content: '›';
    font-size: 1rem;
    font-weight: 400;
    transition: transform var(--transition-fast);
    color: var(--text-muted);
}

.topic-header.expanded::after {
    transform: rotate(90deg);
}

.subtopic-list {
    display: none;
    padding: 4px 8px 8px;
}

.subtopic-list.active {
    display: block;
}

.subtopic-link {
    display: block;
    padding: 8px 12px 8px 28px;
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 0.9rem;
    border-radius: var(--border-radius);
    transition: all var(--transition-fast);
    border-left: 2px solid transparent;
    margin: 2px 0;
}

.subtopic-link:hover {
    background-color: var(--bg-hover);
    color: var(--text-primary);
}

.subtopic-link.active-link {
    background-color: var(--bg-active);
    color: var(--accent-primary);
    border-left-color: var(--accent-primary);
    font-weight: 500;
}

/* --- Main Content Styles --- */
.main-content {
    flex-grow: 1;
    padding: var(--content-padding);
    margin-left: var(--sidebar-width);
    transition: margin-left var(--transition-normal);
    min-height: 100vh;
    max-width: calc(var(--content-max-width) + var(--sidebar-width) + var(--content-padding) * 2);
}

.sidebar.collapsed + .main-content {
    margin-left: var(--sidebar-collapsed-width);
}

/* --- Typography --- */
h1 {
    font-size: 2.5rem;
    font-weight: 700;
    letter-spacing: -0.02em;
    margin-bottom: 0.75rem;
    color: var(--text-primary);
    line-height: 1.2;
}

h1::after {
    content: '';
    display: block;
    width: 60px;
    height: 4px;
    background: linear-gradient(90deg, var(--accent-primary), var(--accent-secondary));
    border-radius: 2px;
    margin-top: 16px;
}

h2 {
    font-size: 1.5rem;
    font-weight: 600;
    letter-spacing: -0.01em;
    margin-top: 0;
    margin-bottom: 1rem;
    color: var(--text-primary);
    display: flex;
    align-items: center;
    gap: 10px;
}

h3 {
    font-size: 1.15rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-top: 1.5rem;
    margin-bottom: 0.75rem;
}

p {
    color: var(--text-secondary);
    margin-bottom: 1rem;
    line-height: 1.75;
}

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

a:hover {
    color: var(--accent-hover);
    text-decoration: underline;
}

strong {
    color: var(--text-primary);
    font-weight: 600;
}

code {
    font-family: var(--font-mono);
    font-size: 0.9em;
    background-color: var(--bg-tertiary);
    padding: 2px 6px;
    border-radius: 4px;
    color: var(--accent-hover);
}

/* --- Resource Section Styles --- */
.resource-section {
    background-color: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius-lg);
    padding: 28px;
    margin: 24px 0;
    transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.resource-section:hover {
    border-color: var(--border-color);
    box-shadow: 0 4px 12px var(--shadow-color);
}

.resource-section h2 {
    padding-bottom: 12px;
    border-bottom: 1px solid var(--border-color);
    margin-bottom: 16px;
}

.resource-section ul, .resource-section ol {
    padding-left: 20px;
    margin: 0;
}

.resource-section li {
    margin-bottom: 10px;
    color: var(--text-secondary);
    line-height: 1.7;
}

.resource-section li strong {
    color: var(--text-primary);
}

.resource-section li::marker {
    color: var(--accent-primary);
}

/* --- Resource Links (Buttons) --- */
.resource-link {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    color: var(--accent-primary);
    text-decoration: none;
    padding: 10px 18px;
    margin: 6px 8px 6px 0;
    border: 1px solid var(--accent-primary);
    border-radius: var(--border-radius);
    font-weight: 500;
    font-size: 0.9rem;
    transition: all var(--transition-fast);
}

.resource-link:hover {
    background-color: var(--accent-primary);
    color: var(--bg-primary);
    text-decoration: none;
    transform: translateY(-1px);
    box-shadow: 0 4px 12px var(--accent-glow);
}

/* --- Responsive Design --- */
@media (max-width: 1024px) {
    .main-content {
        padding: 32px;
    }
    
    h1 {
        font-size: 2rem;
    }
}

@media (max-width: 768px) {
    .sidebar {
        transform: translateX(-100%);
        width: var(--sidebar-width);
    }
    
    .sidebar.mobile-open {
        transform: translateX(0);
    }
    
    .sidebar .topic-header,
    .sidebar .subtopic-list,
    .sidebar h3,
    .sidebar .nav-link {
        display: block;
    }
    
    .main-content {
        margin-left: 0;
        padding: 24px 20px;
    }
    
    h1 {
        font-size: 1.75rem;
    }
    
    .resource-section {
        padding: 20px;
    }
}

/* --- Utility Classes --- */
.text-center { text-align: center; }
.text-muted { color: var(--text-muted); }

.flex { display: flex; }
.flex-wrap { flex-wrap: wrap; }
.items-center { align-items: center; }
.justify-between { justify-content: space-between; }
.gap-1 { gap: 0.5rem; }
.gap-2 { gap: 1rem; }
.gap-3 { gap: 1.5rem; }

.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.mt-3 { margin-top: 1.5rem; }
.mt-4 { margin-top: 2rem; }
.mb-1 { margin-bottom: 0.5rem; }
.mb-2 { margin-bottom: 1rem; }
.mb-3 { margin-bottom: 1.5rem; }
.mb-4 { margin-bottom: 2rem; }

/* --- Animation Utilities --- */
@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

.animate-fade-in {
    animation: fadeIn 0.4s ease-out;
}

/* --- Print Styles --- */
@media print {
    .sidebar { display: none; }
    .main-content { margin-left: 0; }
}