:root {
  --bg: #f6f8fb;
  --card: #ffffff;
  --text: #1a2235;
  --muted: #5b667a;
  --primary: #0b5bd3;
  --secondary: #0aa1ff;
  --border: #e4e8f0;
  --radius: 16px;
  --shadow: 0 8px 28px rgba(0, 0, 0, 0.05);
  --transition: all 0.3s ease;
}

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

body {
  margin: 0;
  font-family: "Inter", system-ui, -apple-system, Segoe UI, Roboto, Ubuntu, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.7;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

/* 🌟 Header */
header {
  background: linear-gradient(135deg, var(--primary), var(--secondary));
  color: #fff;
  padding: 60px 20px 70px;
  text-align: center;
  border-bottom: 4px solid rgba(255, 255, 255, 0.15);
  box-shadow: inset 0 -4px 12px rgba(0, 0, 0, 0.1);
}

.wrap {
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 16px;
}

.title {
  margin: 0 0 8px;
  font-size: clamp(28px, 3vw, 42px);
  font-weight: 800;
  letter-spacing: -0.02em;
}

.sub {
  margin: 0;
  opacity: 0.95;
  font-size: clamp(15px, 1.5vw, 18px);
  font-weight: 400;
}

.meta {
  margin-top: 12px;
  font-size: 14px;
  opacity: 0.85;
}

/* 🌟 Layout */
main {
  margin-top: -40px;
  padding-bottom: 64px;
}

.layout {
  display: grid;
  grid-template-columns: 280px 1fr;
  gap: 24px;
}

/* 🌟 Card Style */
.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  transition: var(--transition);
}

.card:hover {
  transform: translateY(-2px);
  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.08);
}

/* 🌟 Sidebar Navigation */
nav.card {
  position: sticky;
  top: 16px;
  height: max-content;
  padding: 16px;
}

.toc h3 {
  margin: 8px 10px 6px;
  font-size: 14px;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.toc a {
  display: block;
  padding: 10px 14px;
  margin: 5px 0;
  border-radius: 10px;
  text-decoration: none;
  color: var(--text);
  font-size: 15px;
  font-weight: 500;
  transition: var(--transition);
}

.toc a:hover {
  background: #eef4ff;
  color: var(--primary);
  border: 1px solid #d7e2ff;
  padding-left: 22px;
}

/* 🌟 Article Content */
article.card {
  padding: 28px;
}

h2 {
  font-size: 22px;
  margin: 28px 0 12px;
  color: var(--text);
}

p,
li {
  color: #2a3342;
  margin-bottom: 12px;
}

ul {
  margin-left: 20px;
}

/* 🌟 Dividers and Muted Text */
.divider {
  height: 1px;
  background: var(--border);
  margin: 22px 0;
}

.muted {
  color: var(--muted);
  font-size: 14px;
}

/* 🌟 Footer */
.footer {
  text-align: center;
  font-size: 13.5px;
  color: var(--muted);
  padding: 32px 0 16px;
  border-top: 1px solid var(--border);
  background: #fff;
}

/* 🌟 Responsive */
@media (max-width: 980px) {
  .layout {
    grid-template-columns: 1fr;
  }
  nav.card {
    position: static;
  }
  article.card {
    padding: 22px;
  }
}