:root {
    --bg-body: #ffffff;
    --bg-sidebar: #f7f9fc;
    --bg-card: #ffffff;
    --bg-code: #f8fafc;
    /* Light background for code */

    --text-primary: #0f172a;
    --text-secondary: #475569;
    --text-tertiary: #94a3b8;

    --accent-primary: #3b82f6;
    /* Bright Blue */
    --accent-hover: #2563eb;
    --accent-light: #eff6ff;

    --border-color: #e2e8f0;
    --border-hover: #cbd5e1;

    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);

    --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    --font-mono: 'JetBrains Mono', 'Fira Code', monospace;
}

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

body {
    font-family: var(--font-sans);
    background-color: var(--bg-body);
    color: var(--text-primary);
    line-height: 1.6;
    font-size: 16px;
    -webkit-font-smoothing: antialiased;
}

/* Layout Grid */
.layout-wrapper {
    display: grid;
    grid-template-columns: 280px 1fr;
    /* Sidebar | Content */
    max-width: 1400px;
    margin: 0 auto;
    min-height: 100vh;
}

/* Header */
header {
    grid-column: 1 / -1;
    background: var(--bg-body);
    border-bottom: 1px solid var(--border-color);
    padding: 2rem 0;
    text-align: center;
    position: sticky;
    top: 0;
    z-index: 100;
    backdrop-filter: blur(8px);
    background-color: rgba(255, 255, 255, 0.9);
}

h1 {
    font-size: 2.25rem;
    font-weight: 800;
    letter-spacing: -0.025em;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.subtitle {
    color: var(--text-secondary);
    font-size: 1.1rem;
}

.author {
    color: var(--text-tertiary);
    font-size: 0.9rem;
    margin-top: 0.5rem;
}

/* Sidebar */
.sidebar {
    border-right: 1px solid var(--border-color);
    background: var(--bg-body);
    display: none;
    /* Hidden on mobile by default */
}

@media (min-width: 1024px) {
    .sidebar {
        display: block;
    }
}

.sidebar-sticky {
    position: sticky;
    top: 140px;
    /* Below header */
    padding: 2rem;
    max-height: calc(100vh - 140px);
    overflow-y: auto;
}

.sidebar h3 {
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-tertiary);
    margin-bottom: 1rem;
    font-weight: 700;
}

.toc-nav ul {
    list-style: none;
}

.toc-nav li {
    margin-bottom: 0.5rem;
}

.toc-nav a {
    display: block;
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 0.9rem;
    padding: 0.25rem 0;
    transition: color 0.2s;
    border-left: 2px solid transparent;
    padding-left: 1rem;
    margin-left: -1rem;
}

.toc-nav a:hover,
.toc-nav a.active {
    color: var(--accent-primary);
    border-left-color: var(--accent-primary);
}

/* Main Content */
.main-content {
    padding: 3rem 4rem;
    max-width: 900px;
    /* Optimal reading width */
    margin: 0 auto;
}

/* Typography & Elements */
h2 {
    font-size: 1.8rem;
    font-weight: 700;
    letter-spacing: -0.025em;
    margin-top: 3rem;
    margin-bottom: 1.5rem;
    padding-bottom: 0.5rem;
    border-bottom: 1px solid var(--border-color);
    color: var(--text-primary);
}

h3 {
    font-size: 1.4rem;
    font-weight: 600;
    margin-top: 2.5rem;
    margin-bottom: 1rem;
    color: var(--text-primary);
}

h4 {
    font-size: 1.1rem;
    font-weight: 600;
    margin-top: 1.5rem;
    margin-bottom: 0.75rem;
}

p {
    margin-bottom: 1.25rem;
    color: var(--text-secondary);
}

ul,
ol {
    margin-bottom: 1.5rem;
    padding-left: 1.5rem;
    color: var(--text-secondary);
}

li {
    margin-bottom: 0.5rem;
}

/* Cards (Categories, Questions) */
.question-category,
.category-box,
.predicted-question,
.question-type,
.practice-question {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 2rem;
    margin: 2rem 0;
    transition: all 0.2s ease;
}

.question-category:hover,
.category-box:hover {
    border-color: var(--accent-primary);
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}

/* Info Boxes */
.info-box,
.stats-box {
    background: var(--accent-light);
    border: 1px solid #dbeafe;
    border-radius: 12px;
    padding: 1.5rem;
    color: #1e40af;
}

.info-box h3,
.stats-box h3 {
    color: #1e3a8a;
    margin-top: 0;
}

/* Code Blocks */
.code-container {
    background: var(--bg-code);
    border-radius: 8px;
    margin: 1.5rem 0;
    overflow: hidden;
    border: 1px solid var(--border-color);
}

pre {
    padding: 1.5rem;
    overflow-x: auto;
    color: #334155;
    /* Dark slate text for code */
    font-family: var(--font-mono);
    font-size: 0.9rem;
}

/* Buttons */
.action-buttons {
    display: flex;
    justify-content: center;
    gap: 1rem;
    margin-top: 1.5rem;
}

.btn {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.6rem 1.2rem;
    border-radius: 8px;
    font-weight: 500;
    font-size: 0.95rem;
    text-decoration: none;
    transition: all 0.2s;
    cursor: pointer;
}

.btn-secondary {
    background: white;
    border: 1px solid var(--border-color);
    color: var(--text-primary);
    box-shadow: var(--shadow-sm);
}

.btn-secondary:hover {
    border-color: var(--text-tertiary);
    background: #f8fafc;
}

.btn-primary,
.btn-check-answer {
    background: var(--accent-primary);
    color: white;
    border: 1px solid var(--accent-primary);
    box-shadow: var(--shadow-sm);
}

.btn-primary:hover,
.btn-check-answer:hover {
    background: var(--accent-hover);
    transform: translateY(-1px);
}

/* Answer Box */
.answer-box {
    background: #f8fafc;
    border: 1px solid var(--border-color);
    border-left: 4px solid var(--accent-primary);
    border-radius: 8px;
    padding: 1.5rem;
    margin-top: 1.5rem;
}

/* Iframe Overlay */
.iframe-container {
    position: fixed;
    inset: 0;
    background: rgba(0, 0, 0, 0.5);
    backdrop-filter: blur(4px);
    z-index: 1000;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 2rem;
}

.iframe-container iframe {
    width: 100%;
    max-width: 1200px;
    height: 90vh;
    border-radius: 12px;
    border: none;
    background: white;
    box-shadow: 0 25px 50px -12px rgb(0 0 0 / 0.25);
}

.btn-close {
    position: absolute;
    top: 1rem;
    right: 1rem;
    background: white;
    border: none;
    padding: 0.5rem 1rem;
    border-radius: 6px;
    cursor: pointer;
    font-weight: 600;
    box-shadow: var(--shadow-md);
}

/* Footer */
footer {
    grid-column: 1 / -1;
    text-align: center;
    padding: 4rem 0;
    border-top: 1px solid var(--border-color);
    color: var(--text-tertiary);
    background: var(--bg-body);
}

/* Responsive */
@media (max-width: 1024px) {
    .layout-wrapper {
        grid-template-columns: 1fr;
    }

    .main-content {
        padding: 2rem 1.5rem;
    }
}