/* ============================================================
   NETWORKING MASTERY · Stripe-like Clean Light Theme
   Cảm hứng: Stripe Docs / Linear — sáng, sạch, contrast thấp,
   dịu mắt khi đọc lâu. Indigo accent.
   ============================================================ */

:root {
  /* Background — light scale */
  --bg:           #ffffff;
  --bg-soft:      #f8fafc;
  --bg-elev:      #f1f5f9;
  --bg-card:      #ffffff;
  --bg-code:      #f6f8fa;
  --bg-deep:      #e2e8f0;

  /* Borders */
  --border:       #e5e7eb;
  --border-soft:  #f0f2f5;
  --border-strong:#d1d5db;

  /* Text */
  --text:         #1f2937;
  --text-soft:    #475569;
  --text-dim:     #64748b;
  --text-bright:  #0f172a;

  /* Accent — Stripe-like indigo */
  --accent:       #6366f1;
  --accent-deep:  #4f46e5;
  --accent-soft:  rgba(99, 102, 241, 0.10);
  --link:         #4f46e5;
  --link-hover:   #4338ca;

  /* Semantic */
  --green:        #059669;
  --green-soft:   rgba(5, 150, 105, 0.08);
  --yellow:       #d97706;
  --yellow-soft:  rgba(217, 119, 6, 0.08);
  --red:          #dc2626;
  --red-soft:     rgba(220, 38, 38, 0.07);
  --violet:       #7c3aed;
  --violet-soft:  rgba(124, 58, 237, 0.07);
  --cyan:         #0891b2;
  --cyan-soft:    rgba(8, 145, 178, 0.07);
}

* { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, system-ui, sans-serif;
  font-size: 15.5px;
  line-height: 1.7;
  min-height: 100vh;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-rendering: optimizeLegibility;
}

main, header, footer { position: relative; z-index: 1; }

code, kbd, pre, .mono, .ascii { font-family: 'JetBrains Mono', 'IBM Plex Mono', 'Menlo', monospace; }

/* ===== Header ===== */
header {
  background: rgba(255, 255, 255, 0.92);
  border-bottom: 1px solid var(--border);
  padding: 14px 32px;
  position: sticky;
  top: 0;
  z-index: 100;
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
}

nav {
  max-width: 1280px;
  margin: 0 auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 16px;
}

.logo {
  font-family: 'JetBrains Mono', monospace;
  font-weight: 700;
  font-size: 17px;
  color: var(--text-bright);
  letter-spacing: -0.02em;
  display: flex;
  align-items: center;
  gap: 8px;
}
.logo-mark {
  color: var(--accent);
  font-size: 13px;
}
.logo-mark::before { content: '◆'; }

nav ul {
  list-style: none;
  display: flex;
  gap: 22px;
  flex-wrap: wrap;
  align-items: center;
}

nav a {
  color: var(--text-soft);
  text-decoration: none;
  font-size: 14px;
  font-weight: 500;
  transition: color 0.15s ease;
}
nav a:hover, nav a.active {
  color: var(--accent);
}

/* ===== Main ===== */
main {
  max-width: 1100px;
  margin: 0 auto;
  padding: 48px 32px 96px;
}

/* ===== Hero ===== */
.hero {
  margin: 32px 0 56px;
  padding: 52px 44px;
  background: linear-gradient(135deg, #fafbff 0%, #f1f5ff 100%);
  border: 1px solid var(--border);
  border-radius: 12px;
  position: relative;
}

.badge {
  display: inline-block;
  padding: 5px 12px;
  background: var(--accent-soft);
  color: var(--accent-deep);
  border: 1px solid rgba(99, 102, 241, 0.25);
  border-radius: 4px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  font-weight: 600;
  letter-spacing: 0.08em;
  margin-bottom: 24px;
  text-transform: uppercase;
}

.hero h1 {
  font-size: clamp(34px, 5.5vw, 50px);
  font-weight: 800;
  line-height: 1.12;
  letter-spacing: -0.025em;
  color: var(--text-bright);
  margin-bottom: 18px;
}

.text-accent { color: var(--accent); }
.text-blue   { color: var(--link); }
.text-cyan   { color: var(--cyan); }
.text-green  { color: var(--green); }
.text-yellow { color: var(--yellow); }
.text-red    { color: var(--red); }
.text-violet { color: var(--violet); }
.text-orange { color: var(--yellow); }
.text-amber  { color: var(--yellow); }
.text-dim    { color: var(--text-dim); }

.hero .lead {
  font-size: 16.5px;
  color: var(--text-soft);
  max-width: 760px;
  line-height: 1.75;
}

/* ===== Headings ===== */
h2 {
  font-size: 26px;
  font-weight: 700;
  letter-spacing: -0.02em;
  margin: 56px 0 22px;
  color: var(--text-bright);
  display: flex;
  align-items: center;
  gap: 12px;
  padding-bottom: 10px;
  border-bottom: 1px solid var(--border);
}
h2::before {
  content: '#';
  color: var(--accent);
  font-family: 'JetBrains Mono', monospace;
  font-weight: 700;
  font-size: 24px;
}

h3 {
  font-size: 19px;
  font-weight: 700;
  margin: 30px 0 12px;
  color: var(--text-bright);
}

h4 {
  font-size: 13px;
  font-weight: 600;
  margin: 20px 0 10px;
  color: var(--accent-deep);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  font-family: 'JetBrains Mono', monospace;
}

p { margin: 12px 0; color: var(--text); }
ul, ol { margin: 12px 0 12px 24px; }
li { margin: 6px 0; color: var(--text); }

a { color: var(--link); text-decoration: none; }
a:hover { color: var(--link-hover); text-decoration: underline; }

strong { color: var(--text-bright); font-weight: 700; }
em { color: var(--violet); font-style: italic; }

/* ===== Inline code ===== */
:not(pre) > code {
  background: var(--bg-code);
  color: #be185d;
  padding: 2px 7px;
  border-radius: 4px;
  border: 1px solid var(--border);
  font-size: 0.88em;
  font-weight: 500;
}

/* ===== Code blocks ===== */
pre {
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 18px 22px 22px;
  margin: 18px 0;
  overflow-x: auto;
  position: relative;
}
pre::before {
  content: attr(data-lang);
  position: absolute;
  top: 6px;
  left: 14px;
  font-size: 10.5px;
  color: var(--text-dim);
  font-family: 'JetBrains Mono', monospace;
  text-transform: uppercase;
  letter-spacing: 0.08em;
}
pre code {
  display: block;
  margin-top: 10px;
  color: var(--text);
  background: none;
  border: none;
  padding: 0;
  font-size: 13px;
  line-height: 1.7;
  font-weight: 500;
}

.copy-btn {
  position: absolute;
  top: 6px;
  right: 14px;
  background: var(--bg);
  border: 1px solid var(--border);
  color: var(--text-dim);
  padding: 3px 10px;
  font-size: 11px;
  font-family: 'JetBrains Mono', monospace;
  border-radius: 4px;
  cursor: pointer;
  transition: all 0.15s ease;
  opacity: 0;
}
pre:hover .copy-btn { opacity: 1; }
.copy-btn:hover { color: var(--accent); border-color: var(--accent); }
.copy-btn.copied { color: var(--green); border-color: var(--green); }

/* ===== Callouts ===== */
.callout {
  margin: 22px 0;
  padding: 16px 20px;
  border-left: 3px solid var(--accent);
  background: var(--accent-soft);
  border-radius: 0 6px 6px 0;
}
.callout-title {
  font-weight: 700;
  margin-bottom: 6px;
  color: var(--accent-deep);
  font-family: 'JetBrains Mono', monospace;
  font-size: 12.5px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}
.callout.tip      { border-left-color: var(--green); background: var(--green-soft); }
.callout.tip .callout-title { color: var(--green); }
.callout.warning  { border-left-color: var(--yellow); background: var(--yellow-soft); }
.callout.warning .callout-title { color: var(--yellow); }
.callout.danger   { border-left-color: var(--red); background: var(--red-soft); }
.callout.danger .callout-title { color: var(--red); }
.callout.note     { border-left-color: var(--violet); background: var(--violet-soft); }
.callout.note .callout-title { color: var(--violet); }

/* ===== Roadmap cards ===== */
.roadmap {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 14px;
  margin: 24px 0;
}

.card {
  display: block;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 18px 22px;
  text-decoration: none;
  color: var(--text);
  transition: all 0.18s ease;
  position: relative;
}
.card:hover {
  border-color: var(--accent);
  background: var(--bg-soft);
  transform: translateY(-1px);
  text-decoration: none;
  box-shadow: 0 8px 20px rgba(99, 102, 241, 0.10);
}

.card .ch-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 10.5px;
  font-weight: 700;
  color: var(--accent);
  letter-spacing: 0.1em;
  margin-bottom: 8px;
  display: block;
}
.card h3 {
  font-size: 16px;
  margin: 0 0 6px;
  color: var(--text-bright);
}
.card p {
  font-size: 13px;
  color: var(--text-soft);
  margin: 0;
  line-height: 1.55;
}

/* ===== Tables ===== */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 18px 0;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  overflow: hidden;
}
th, td {
  padding: 11px 16px;
  text-align: left;
  border-bottom: 1px solid var(--border-soft);
  color: var(--text);
}
th {
  background: var(--bg-soft);
  color: var(--accent-deep);
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-weight: 700;
}
tr:last-child td { border-bottom: none; }
tr:hover td { background: var(--bg-soft); }

/* ===== Lesson navigation ===== */
.lesson-nav {
  display: flex;
  justify-content: space-between;
  gap: 14px;
  margin: 56px 0 24px;
  flex-wrap: wrap;
}
.lesson-nav a {
  flex: 1;
  min-width: 240px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 16px 20px;
  text-decoration: none;
  color: var(--text-bright);
  transition: all 0.18s ease;
  display: flex;
  flex-direction: column;
  gap: 4px;
}
.lesson-nav a:hover {
  border-color: var(--accent);
  background: var(--bg-soft);
  text-decoration: none;
  box-shadow: 0 6px 14px rgba(99, 102, 241, 0.10);
}
.lesson-nav a .label {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

/* ===== Footer ===== */
footer {
  text-align: center;
  padding: 32px;
  color: var(--text-dim);
  font-size: 13px;
  border-top: 1px solid var(--border);
  margin-top: 48px;
  font-family: 'JetBrains Mono', monospace;
}
.footer-mark { color: var(--accent); }

/* ===== Syllabus ===== */
.syllabus { margin: 32px 0; }

.chapter-doc {
  margin: 24px 0;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  overflow: hidden;
}

.chapter-doc-head {
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 14px 22px;
  background: var(--bg-soft);
  border-bottom: 1px solid var(--border);
}
.chapter-doc-head .ch-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  font-weight: 700;
  color: var(--accent-deep);
  background: var(--accent-soft);
  padding: 4px 10px;
  border-radius: 4px;
  border: 1px solid rgba(99, 102, 241, 0.25);
  letter-spacing: 0.05em;
  white-space: nowrap;
}
.chapter-doc-head h3 {
  margin: 0;
  flex: 1;
  font-size: 18px;
  color: var(--text-bright);
}
.chapter-doc-head .ch-tag {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  color: var(--cyan);
  background: var(--cyan-soft);
  padding: 4px 10px;
  border-radius: 4px;
  border: 1px solid rgba(8, 145, 178, 0.2);
}

.chapter-doc-body { padding: 18px 26px 22px; }

.ch-summary {
  font-style: italic;
  color: var(--text-soft);
  border-left: 2px solid var(--accent);
  padding-left: 14px;
  margin: 8px 0 18px;
}

/* ===== Stats ===== */
.stats {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 14px;
  margin: 24px 0;
}
.stat {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-left: 3px solid var(--accent);
  border-radius: 0 6px 6px 0;
  padding: 14px 18px;
}
.stat-value {
  font-family: 'JetBrains Mono', monospace;
  font-size: 24px;
  font-weight: 700;
  color: var(--text-bright);
  line-height: 1;
  margin-bottom: 4px;
}
.stat-label {
  font-size: 11.5px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

/* ===== Compare ===== */
.compare {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 14px;
  margin: 20px 0;
}
.compare > div {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 16px 18px;
}
.compare > .good { border-top: 3px solid var(--green); }
.compare > .bad  { border-top: 3px solid var(--red); }
.compare h4 { margin-top: 0; }

@media (max-width: 768px) {
  .compare { grid-template-columns: 1fr; }
}

/* ===== TOC sidebar ===== */
.chapter-layout {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 36px;
  align-items: start;
}
.chapter-toc {
  position: sticky;
  top: 80px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 16px;
  font-size: 13px;
}
.chapter-toc h4 { margin-top: 0; }
.chapter-toc ul { list-style: none; margin: 0; padding: 0; }
.chapter-toc li { margin: 3px 0; }
.chapter-toc a {
  color: var(--text-soft);
  display: block;
  padding: 4px 8px;
  border-left: 2px solid transparent;
  transition: all 0.15s ease;
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
}
.chapter-toc a:hover {
  color: var(--accent);
  border-left-color: var(--accent);
  background: var(--bg-soft);
  text-decoration: none;
}

@media (max-width: 900px) {
  .chapter-layout { grid-template-columns: 1fr; }
  .chapter-toc { position: static; }
}

/* ===== Quiz ===== */
.quiz {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 22px;
  margin: 32px 0;
}
.quiz h3 {
  margin-top: 0;
  color: var(--accent-deep);
}
.quiz-question {
  margin: 18px 0;
  padding: 14px 18px;
  background: var(--bg-soft);
  border-left: 3px solid var(--accent);
  border-radius: 0 6px 6px 0;
}
.quiz-question p { margin-bottom: 10px; font-weight: 500; color: var(--text-bright); }
.quiz-options { list-style: none; margin: 0; padding: 0; }
.quiz-options li {
  margin: 6px 0;
  padding: 9px 14px;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.15s ease;
  color: var(--text);
}
.quiz-options li:hover {
  border-color: var(--accent);
  background: var(--bg-soft);
}
.quiz-options li.correct {
  background: var(--green-soft);
  border-color: var(--green);
  color: var(--green);
}
.quiz-options li.wrong {
  background: var(--red-soft);
  border-color: var(--red);
  color: var(--red);
}
.quiz-options li.disabled { pointer-events: none; opacity: 0.7; }

.quiz-explain {
  margin-top: 10px;
  font-size: 13px;
  color: var(--text-soft);
  font-style: italic;
  display: none;
}
.quiz-question.answered .quiz-explain { display: block; }

/* ===== Exercise ===== */
.exercise {
  background: var(--bg-card);
  border: 1px dashed var(--cyan);
  border-radius: 8px;
  padding: 16px 20px;
  margin: 18px 0;
}
.exercise-title {
  font-family: 'JetBrains Mono', monospace;
  font-size: 12px;
  font-weight: 700;
  color: var(--cyan);
  letter-spacing: 0.05em;
  text-transform: uppercase;
  margin-bottom: 8px;
}

/* ===== ASCII art / diagrams ===== */
.ascii-art {
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 18px 22px;
  margin: 18px 0;
  font-family: 'JetBrains Mono', monospace;
  font-size: 12.5px;
  line-height: 1.5;
  color: var(--text-bright);
  overflow-x: auto;
  white-space: pre;
}

/* ===== Network / packet tags ===== */
.layer-tag, .protocol-tag {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 3px 10px;
  border-radius: 4px;
  font-family: 'JetBrains Mono', monospace;
  font-size: 11.5px;
  font-weight: 600;
  border: 1px solid currentColor;
  margin: 0 2px;
  background: var(--bg-soft);
}
.layer-tag.l7 { color: var(--violet); }
.layer-tag.l6 { color: var(--cyan); }
.layer-tag.l5 { color: var(--green); }
.layer-tag.l4 { color: var(--accent); }
.layer-tag.l3 { color: var(--yellow); }
.layer-tag.l2 { color: var(--red); }
.layer-tag.l1 { color: var(--text-dim); }

/* ===== Chapter Jump Menu ===== */
.ch-jump {
  position: relative;
  list-style: none;
}
.ch-jump-btn {
  background: var(--accent-soft);
  border: 1px solid rgba(99, 102, 241, 0.3);
  color: var(--accent-deep);
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  font-weight: 600;
  padding: 5px 12px;
  border-radius: 4px;
  cursor: pointer;
  transition: all 0.15s ease;
  letter-spacing: 0.04em;
}
.ch-jump-btn:hover {
  background: rgba(99, 102, 241, 0.15);
  color: var(--accent-deep);
}
.ch-jump-btn[aria-expanded="true"] {
  background: rgba(99, 102, 241, 0.18);
}

.ch-jump-menu {
  position: absolute;
  top: calc(100% + 8px);
  right: 0;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 8px;
  min-width: 380px;
  max-width: 90vw;
  max-height: min(70vh, 600px);
  overflow-y: auto;
  display: none;
  z-index: 1000;
  box-shadow: 0 16px 40px rgba(15, 23, 42, 0.12), 0 0 0 1px rgba(99, 102, 241, 0.05);
  padding: 8px;
}
.ch-jump-menu.open { display: block; animation: ch-jump-fadein 0.15s ease; }

@keyframes ch-jump-fadein {
  from { opacity: 0; transform: translateY(-6px); }
  to   { opacity: 1; transform: translateY(0); }
}

.ch-jump-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 8px 12px 12px;
  border-bottom: 1px solid var(--border-soft);
  margin-bottom: 6px;
}
.ch-jump-title {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--text-dim);
}
.ch-jump-home {
  font-size: 12px;
  color: var(--accent);
  text-decoration: none;
}
.ch-jump-home:hover { color: var(--accent-deep); text-decoration: underline; }

.ch-jump-item {
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 10px 12px;
  border-radius: 6px;
  text-decoration: none;
  color: var(--text);
  transition: background 0.12s ease;
}
.ch-jump-item:hover {
  background: var(--bg-soft);
  text-decoration: none;
}
.ch-jump-item.active {
  background: var(--accent-soft);
  border-left: 3px solid var(--accent);
  padding-left: 9px;
}

.ch-jump-num {
  font-family: 'JetBrains Mono', monospace;
  font-size: 11px;
  font-weight: 700;
  color: var(--accent-deep);
  background: var(--accent-soft);
  border: 1px solid rgba(99, 102, 241, 0.25);
  padding: 4px 8px;
  border-radius: 4px;
  letter-spacing: 0.04em;
  flex-shrink: 0;
}

.ch-jump-text {
  display: flex;
  flex-direction: column;
  gap: 2px;
  min-width: 0;
}
.ch-jump-name {
  font-family: 'Inter', sans-serif;
  font-size: 13.5px;
  font-weight: 500;
  color: var(--text-bright);
}
.ch-jump-tag {
  font-family: 'JetBrains Mono', monospace;
  font-size: 10.5px;
  color: var(--text-dim);
  letter-spacing: 0.04em;
}

@media (max-width: 600px) {
  .ch-jump-menu { right: -16px; min-width: 320px; }
}

/* ===== Responsive ===== */
@media (max-width: 768px) {
  main { padding: 32px 20px 64px; }
  .hero { padding: 36px 24px; }
  .hero h1 { font-size: 30px; }
  nav { padding: 0; }
  nav ul { gap: 14px; }
  .lesson-nav a { min-width: 100%; }
}

.center { text-align: center; }
.mt-2 { margin-top: 14px; }