:root {
    --primary: #0f172a;
    --secondary: #334155;
    --accent: #2563eb;
    --bg-light: #f8fafc;
    --text-main: #1e293b;
    --text-muted: #64748b;
    --sidebar-bg: #ffffff;
    --sidebar-border: #e2e8f0;
}

body {
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
    line-height: 1.6;
    color: var(--text-main);
    background: var(--bg-light);
    margin: 0;
    display: flex;
    min-height: 100vh;
}

/* Sidebar */
.sidebar {
    width: 260px;
    background: var(--sidebar-bg);
    border-right: 1px solid var(--sidebar-border);
    padding: 2rem 1.5rem;
    position: fixed;
    height: 100vh;
    overflow-y: auto;
    box-sizing: border-box;
}

.brand {
    font-size: 1.25rem;
    font-weight: 800;
    color: var(--primary);
    margin-bottom: 2rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    border-bottom: 2px solid var(--sidebar-border);
    padding-bottom: 1rem;
}

.nav-links {
    list-style: none;
    padding: 0;
    margin: 0;
}

.nav-links li {
    margin-bottom: 0.2rem;
}

.nav-links a {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.6rem 1rem;
    color: var(--secondary);
    text-decoration: none;
    border-radius: 0.4rem;
    transition: all 0.2s;
    font-weight: 500;
    font-size: 0.95rem;
}

.nav-links a:hover {
    background: #f1f5f9;
    color: var(--primary);
}

.nav-links a.active {
    background: #eff6ff;
    color: var(--accent);
    font-weight: 600;
}

.chapter-title {
    font-size: 0.75rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-muted);
    font-weight: 700;
    margin-top: 1.5rem;
    margin-bottom: 0.5rem;
    padding-left: 1rem;
}

/* Main Content */
.content {
    margin-left: 260px;
    padding: 3rem 4rem;
    max-width: 900px;
    box-sizing: border-box;
    width: 100%;
}

.book-page {
    background: white;
    padding: 3rem 4rem;
    border-radius: 8px;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    min-height: 70vh;
}

h1 {
    font-size: 2.2rem;
    color: var(--primary);
    margin-top: 0;
    margin-bottom: 1rem;
    letter-spacing: -0.02em;
    border-bottom: 2px solid var(--sidebar-border);
    padding-bottom: 1rem;
}

h2 {
    font-size: 1.8rem;
    color: var(--secondary);
    margin-top: 2rem;
    margin-bottom: 1rem;
}

h3 {
    font-size: 1.3rem;
    color: var(--primary);
    margin-top: 2rem;
    margin-bottom: 0.8rem;
}

p {
    margin-bottom: 1.2rem;
    color: #475569;
    font-size: 1.05rem;
}

ul, ol {
    margin-bottom: 1.5rem;
    color: #475569;
    padding-left: 2rem;
}

li {
    margin-bottom: 0.5rem;
    line-height: 1.6;
}

.screenshot {
    max-width: 100%;
    height: auto;
    border-radius: 8px;
    border: 1px solid #e2e8f0;
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    margin: 2rem 0;
}

.nav-buttons {
    display: flex;
    justify-content: space-between;
    margin-top: 4rem;
    padding-top: 2rem;
    border-top: 1px solid var(--sidebar-border);
}

.btn-nav {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.75rem 1.5rem;
    background: white;
    border: 1px solid var(--sidebar-border);
    border-radius: 0.375rem;
    color: var(--secondary);
    text-decoration: none;
    font-weight: 500;
    transition: all 0.2s;
}

.btn-nav:hover {
    background: #f8fafc;
    border-color: #cbd5e1;
    color: var(--primary);
}