:root {
  --bg: #f5f1e8;
  --paper: #fffdf7;
  --ink: #171813;
  --muted: #68685f;
  --line: #d8d3c7;
  --accent: #d7502a;
  --accent-soft: #f3d8cd;
  --green: #2f6f56;
  --code: #20231f;
  --shadow: 0 18px 60px rgba(43, 39, 30, .08);
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body { margin: 0; color: var(--ink); background: var(--bg); font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif; }
button, textarea { font: inherit; }
a { color: inherit; }
.shell { min-height: 100vh; display: grid; grid-template-columns: 300px 1fr; }
.sidebar { position: sticky; top: 0; height: 100vh; padding: 28px 22px; border-right: 1px solid var(--line); display: flex; flex-direction: column; background: rgba(245,241,232,.94); backdrop-filter: blur(14px); }
.brand { display: flex; align-items: center; gap: 12px; }
.brand-mark { width: 42px; height: 42px; border-radius: 12px; display: grid; place-items: center; color: white; background: var(--ink); font-family: ui-monospace, SFMono-Regular, Menlo, monospace; font-weight: 800; }
.brand strong { display: block; font-size: 15px; }
.brand span { display: block; margin-top: 3px; color: var(--muted); font-size: 11px; }
.progress-wrap { margin: 28px 0 20px; }
.progress-copy { display: flex; justify-content: space-between; font-size: 11px; color: var(--muted); margin-bottom: 7px; }
.progress { height: 5px; background: #e0dbcf; overflow: hidden; border-radius: 999px; }
.progress > div { height: 100%; width: 0; background: var(--accent); transition: width .25s ease; }
#lessonNav { display: grid; gap: 4px; overflow: auto; padding-right: 4px; }
.nav-item { width: 100%; border: 0; background: transparent; display: grid; grid-template-columns: 26px 1fr 18px; align-items: center; gap: 9px; text-align: left; padding: 10px 8px; border-radius: 9px; cursor: pointer; color: var(--muted); }
.nav-item:hover, .nav-item.active { background: rgba(23,24,19,.06); color: var(--ink); }
.nav-num { font-family: ui-monospace, monospace; font-size: 10px; }
.nav-title { font-size: 12px; line-height: 1.25; }
.nav-check { width: 15px; height: 15px; border: 1px solid #aaa69d; border-radius: 50%; display: grid; place-items: center; font-size: 9px; }
.nav-item.done .nav-check { background: var(--green); color: white; border-color: var(--green); }
.sidebar-foot { margin-top: auto; display: flex; gap: 12px; padding-top: 20px; font-size: 11px; color: var(--muted); }
.sidebar-foot a:hover { color: var(--accent); }
main { min-width: 0; }
.hero { max-width: 1180px; margin: 0 auto; padding: 78px 56px 42px; display: grid; grid-template-columns: 1.28fr .72fr; gap: 50px; align-items: center; }
.eyebrow { margin: 0 0 14px; font-size: 11px; font-weight: 800; letter-spacing: .15em; color: var(--accent); }
h1 { margin: 0; max-width: 820px; font-family: Georgia, "Times New Roman", serif; font-size: clamp(42px,5.5vw,74px); font-weight: 500; line-height: .98; letter-spacing: -.035em; }
.lede { max-width: 770px; margin: 26px 0 0; color: var(--muted); font-size: 17px; line-height: 1.65; }
.hero-actions { display: flex; gap: 10px; margin-top: 28px; }
button.primary, button.ghost, button.small { border-radius: 10px; padding: 11px 15px; cursor: pointer; transition: transform .15s ease, background .15s ease; }
button.primary { border: 1px solid var(--ink); background: var(--ink); color: white; }
button.ghost, button.small { border: 1px solid var(--line); background: transparent; color: var(--ink); }
button:hover { transform: translateY(-1px); }
.architecture-card { background: var(--ink); color: #f8f4eb; border-radius: 22px; padding: 24px; box-shadow: var(--shadow); }
.mini-label { display: block; margin-bottom: 16px; color: #aaa89f; font-size: 10px; letter-spacing: .16em; }
.arch-row { display: grid; grid-template-columns: 1fr 24px 1fr; align-items: center; gap: 8px; text-align: center; }
.arch-node { padding: 17px 10px; border: 1px solid #45473f; border-radius: 13px; font-size: 11px; line-height: 1.5; color: #b8b8ae; }
.arch-node b { color: white; font-family: ui-monospace, monospace; font-size: 13px; }
.arch-node.accent { border-color: #a1472e; background: #30241f; }
.arch-center { text-align: center; color: #85867e; font-size: 10px; padding: 12px; }
.equation { margin-top: 17px; padding-top: 16px; border-top: 1px solid #35372f; font: 12px ui-monospace, monospace; color: #dddcd4; text-align: center; }
.principles { max-width: 1180px; margin: 0 auto; padding: 12px 56px 56px; display: grid; grid-template-columns: repeat(3,1fr); gap: 1px; }
.principles > div { border-top: 1px solid var(--line); padding: 18px 20px 0 0; }
.principles span { display: block; color: var(--accent); font: 10px ui-monospace, monospace; margin-bottom: 18px; }
.principles b { font-family: Georgia, serif; font-size: 19px; font-weight: 500; }
.principles p { color: var(--muted); font-size: 12px; line-height: 1.55; max-width: 300px; }
.lesson-host { max-width: 1180px; margin: 0 auto; padding: 0 56px 62px; }
.lesson { background: var(--paper); border: 1px solid var(--line); border-radius: 22px; box-shadow: var(--shadow); overflow: hidden; }
.lesson-head { padding: 36px 40px 28px; border-bottom: 1px solid var(--line); display: grid; grid-template-columns: 1fr auto; gap: 20px; align-items: end; }
.lesson-kicker { font: 11px ui-monospace, monospace; color: var(--accent); letter-spacing: .08em; }
.lesson h2 { margin: 7px 0 0; font: 500 36px/1.05 Georgia, serif; }
.lesson-goal { margin: 11px 0 0; max-width: 700px; color: var(--muted); font-size: 14px; line-height: 1.55; }
.duration { padding: 8px 11px; border-radius: 999px; background: #eeeadf; color: var(--muted); font-size: 11px; white-space: nowrap; }
.lesson-body { padding: 34px 40px 40px; display: grid; grid-template-columns: minmax(0, 1fr) 300px; gap: 40px; }
.content h3 { margin: 26px 0 10px; font-size: 15px; }
.content h3:first-child { margin-top: 0; }
.content p, .content li { font-size: 14px; line-height: 1.65; color: #404139; }
.content ul, .content ol { padding-left: 20px; }
.callout { margin: 20px 0; padding: 16px 18px; border-left: 3px solid var(--accent); background: #f8eee9; border-radius: 0 10px 10px 0; font-size: 13px; line-height: 1.55; }
.formula { margin: 18px 0; padding: 15px 18px; background: #f0ede5; border-radius: 10px; font: 13px/1.65 ui-monospace, SFMono-Regular, Menlo, monospace; overflow: auto; }
pre { margin: 18px 0; padding: 18px; background: var(--code); color: #ecebe3; border-radius: 13px; font: 12px/1.55 ui-monospace, SFMono-Regular, Menlo, monospace; overflow-x: auto; }
.sidebar-card { align-self: start; position: sticky; top: 22px; display: grid; gap: 14px; }
.task-card, .quiz-card { border: 1px solid var(--line); border-radius: 14px; padding: 17px; background: #faf7ef; }
.task-card h4, .quiz-card h4 { margin: 0 0 8px; font-size: 12px; }
.task-card p, .quiz-card p { margin: 0; color: var(--muted); font-size: 11px; line-height: 1.5; }
.done-when { margin-top: 12px; padding-top: 12px; border-top: 1px dashed var(--line); }
.done-when b { display: block; margin-bottom: 5px; font-size: 10px; letter-spacing: .08em; color: var(--green); }
.complete-btn { width: 100%; border: 0; border-radius: 10px; padding: 11px; background: var(--green); color: white; cursor: pointer; font-size: 12px; }
.complete-btn.completed { background: #dfe7e1; color: var(--green); }
.lesson-pagination { padding: 0 40px 36px; display: flex; justify-content: space-between; gap: 12px; }
.choice { margin-top: 8px; width: 100%; text-align: left; padding: 9px 10px; border: 1px solid var(--line); border-radius: 8px; background: white; cursor: pointer; font-size: 11px; }
.choice.correct { border-color: var(--green); background: #edf4ef; }
.choice.wrong { border-color: #a7523c; background: #faece8; }
.quiz-feedback { margin-top: 8px !important; font-weight: 600; }
.field-notes { max-width: 1180px; margin: 0 auto; padding: 0 56px 72px; display: grid; grid-template-columns: 300px 1fr; gap: 40px; }
.field-notes h2 { margin: 0; font: 500 32px Georgia, serif; }
.field-notes p:not(.eyebrow) { color: var(--muted); font-size: 12px; line-height: 1.55; }
textarea { min-height: 180px; width: 100%; border: 1px solid var(--line); border-radius: 14px; padding: 16px; resize: vertical; background: var(--paper); color: var(--ink); line-height: 1.55; outline: none; }
textarea:focus { border-color: #aaa399; box-shadow: 0 0 0 3px rgba(0,0,0,.04); }
footer { border-top: 1px solid var(--line); padding: 22px 56px 34px; display: flex; justify-content: space-between; color: var(--muted); font-size: 10px; }
@media (max-width: 980px) {
  .shell { grid-template-columns: 1fr; }
  .sidebar { position: relative; height: auto; border-right: 0; border-bottom: 1px solid var(--line); }
  #lessonNav { grid-template-columns: repeat(3, 1fr); }
  .sidebar-foot { margin-top: 12px; }
  .hero { grid-template-columns: 1fr; padding-top: 48px; }
  .lesson-body { grid-template-columns: 1fr; }
  .sidebar-card { position: static; }
}
@media (max-width: 680px) {
  .hero, .principles, .lesson-host, .field-notes { padding-left: 20px; padding-right: 20px; }
  .principles { grid-template-columns: 1fr; gap: 20px; }
  .lesson-head, .lesson-body { padding-left: 22px; padding-right: 22px; }
  .lesson-pagination { padding-left: 22px; padding-right: 22px; }
  .field-notes { grid-template-columns: 1fr; gap: 14px; }
  #lessonNav { grid-template-columns: 1fr 1fr; }
  footer { padding-left: 20px; padding-right: 20px; flex-direction: column; gap: 8px; }
}