/* ============================================
   PYTHON DESDE CERO — Design System
   A warm editorial technical book aesthetic.
   ============================================ */

@import url('https://fonts.googleapis.com/css2?family=Newsreader:ital,opsz,wght@0,6..72,400;0,6..72,500;0,6..72,600;0,6..72,700;1,6..72,400;1,6..72,500&family=Instrument+Sans:ital,wght@0,400;0,500;0,600;0,700;1,400&family=JetBrains+Mono:ital,wght@0,400;0,500;0,600;0,700;1,400&display=swap');

:root {
  /* Type families */
  --font-display: 'Newsreader', 'Iowan Old Style', Georgia, serif;
  --font-body: 'Instrument Sans', 'Helvetica Neue', system-ui, sans-serif;
  --font-mono: 'JetBrains Mono', 'SF Mono', Menlo, Consolas, monospace;

  /* Light theme — warm paper */
  --paper: #F4EEDF;
  --paper-2: #EAE3D0;     /* slightly darker — code blocks, sidebar */
  --paper-3: #DDD3BB;     /* dividers, deep surface */
  --ink: #1A1715;
  --ink-2: #443D34;       /* secondary text */
  --ink-3: #6F665A;       /* tertiary, captions */
  --border: #C8BE9F;
  --border-soft: #DCD2B5;

  /* Brand accents */
  --accent: #1F4E5F;          /* deep petrol — links, primary */
  --accent-hover: #163E4D;
  --accent-soft: #C9DAE0;
  --highlight: #D9742A;       /* warm coral/burnt orange — callouts, emphasis */
  --highlight-soft: #F2D9BD;
  --amber: #C99419;           /* mustard — quiz/exercise tags */
  --amber-soft: #F1DC9C;
  --warn: #A8331E;            /* errors, "cuidado" */
  --warn-soft: #F0CBC0;
  --ok: #3F6B3B;              /* success, correct */
  --ok-soft: #CDDDB8;

  /* Code syntax (warm light) */
  --code-bg: #2A2520;
  --code-fg: #ECE4D2;
  --code-comment: #897E68;
  --code-keyword: #E8A06B;
  --code-string: #B5C58B;
  --code-number: #D5A8C3;
  --code-builtin: #8FB8C7;
  --code-func: #F0CC72;
  --code-op: #C9C0AB;
  --code-prompt: #6F665A;

  /* Spacing scale */
  --s-1: 4px;
  --s-2: 8px;
  --s-3: 12px;
  --s-4: 16px;
  --s-5: 24px;
  --s-6: 32px;
  --s-7: 48px;
  --s-8: 64px;
  --s-9: 96px;

  /* Radii */
  --r-xs: 2px;
  --r-sm: 4px;
  --r-md: 8px;
  --r-lg: 14px;

  /* Reading width */
  --measure: 68ch;
  --base-size: 18px;
  --line-height: 1.65;

  /* Motion */
  --t-fast: 120ms cubic-bezier(0.3, 0, 0.2, 1);
  --t-base: 200ms cubic-bezier(0.3, 0, 0.2, 1);
}

[data-theme="dark"] {
  --paper: #1A1714;
  --paper-2: #221E1A;
  --paper-3: #2C2722;
  --ink: #F0E9D6;
  --ink-2: #C6BCA4;
  --ink-3: #8F8472;
  --border: #3A332C;
  --border-soft: #2C2722;
  --accent: #79B3C4;
  --accent-hover: #9BCAD8;
  --accent-soft: #2C4750;
  --highlight: #E89858;
  --highlight-soft: #4A3522;
  --amber: #E5BB60;
  --amber-soft: #4A3F1F;
  --warn: #E5826B;
  --warn-soft: #4A2A22;
  --ok: #8FBE82;
  --ok-soft: #2C3E25;
  --code-bg: #0F0D0B;
}

/* ============================================
   Base
   ============================================ */
* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body {
  font-family: var(--font-body);
  font-size: var(--base-size);
  line-height: var(--line-height);
  color: var(--ink);
  background: var(--paper);
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
  font-feature-settings: 'ss01', 'cv11';
}

/* Subtle paper texture — overprinted noise */
body::before {
  content: '';
  position: fixed;
  inset: 0;
  pointer-events: none;
  z-index: 1000;
  opacity: 0.5;
  mix-blend-mode: multiply;
  background-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 200 200' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.85' numOctaves='2' stitchTiles='stitch'/%3E%3CfeColorMatrix values='0 0 0 0 0.1 0 0 0 0 0.09 0 0 0 0 0.07 0 0 0 0.06 0'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
}

[data-theme="dark"] body::before {
  opacity: 0.7;
  mix-blend-mode: screen;
  background-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 200 200' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.85' numOctaves='2' stitchTiles='stitch'/%3E%3CfeColorMatrix values='0 0 0 0 0.95 0 0 0 0 0.9 0 0 0 0 0.8 0 0 0 0.04 0'/%3E%3C/filter%3E%3Crect width='100%25' height='100%25' filter='url(%23n)'/%3E%3C/svg%3E");
}

::selection { background: var(--highlight); color: var(--paper); }

/* ============================================
   Typography primitives
   ============================================ */
h1, h2, h3, h4 {
  font-family: var(--font-display);
  font-weight: 500;
  line-height: 1.12;
  letter-spacing: -0.02em;
  color: var(--ink);
  margin: 0;
  text-wrap: balance;
  font-feature-settings: 'ss01', 'ss02';
}
h1 { font-size: clamp(2.4rem, 4.2vw, 3.6rem); font-weight: 500; letter-spacing: -0.025em; }
h2 { font-size: clamp(1.8rem, 2.6vw, 2.3rem); margin-top: var(--s-8); margin-bottom: var(--s-4); }
h3 { font-size: 1.35rem; margin-top: var(--s-6); margin-bottom: var(--s-3); }
h4 { font-size: 1.05rem; font-family: var(--font-body); font-weight: 600; letter-spacing: 0; text-transform: uppercase; color: var(--ink-3); margin-top: var(--s-5); margin-bottom: var(--s-2); }

p { margin: 0 0 1.1em; text-wrap: pretty; }
p + p { margin-top: 0; }
strong { font-weight: 600; color: var(--ink); }
em { font-style: italic; font-family: var(--font-display); font-weight: 400; }

a {
  color: var(--accent);
  text-decoration: underline;
  text-decoration-thickness: 1px;
  text-underline-offset: 3px;
  transition: color var(--t-fast);
}
a:hover { color: var(--highlight); }

ul, ol { padding-left: 1.4em; margin: 0 0 1.2em; }
li { margin-bottom: 0.35em; }
li::marker { color: var(--ink-3); }

hr { border: 0; border-top: 1px solid var(--border-soft); margin: var(--s-7) 0; }

/* Inline code */
code {
  font-family: var(--font-mono);
  font-size: 0.88em;
  background: var(--paper-2);
  border: 1px solid var(--border-soft);
  padding: 1px 5px;
  border-radius: var(--r-sm);
  color: var(--ink);
  font-feature-settings: 'liga' off;
}

/* ============================================
   Eyebrows & numerals — mono micro labels
   ============================================ */
.eyebrow {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink-3);
  font-weight: 500;
}
.eyebrow-accent { color: var(--highlight); }

.numeral {
  font-family: var(--font-display);
  font-style: italic;
  font-weight: 400;
  color: var(--highlight);
}

/* ============================================
   Layout
   ============================================ */
.app {
  display: grid;
  grid-template-columns: 320px 1fr;
  min-height: 100vh;
}

.sidebar {
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
  background: var(--paper-2);
  border-right: 1px solid var(--border-soft);
  padding: var(--s-6) var(--s-5);
  scrollbar-width: thin;
  scrollbar-color: var(--border) transparent;
}

.reader {
  min-width: 0;
  padding: var(--s-8) var(--s-7);
  display: flex;
  flex-direction: column;
  align-items: center;
}

.reader-inner {
  width: 100%;
  max-width: var(--measure);
}

.reader-inner.wide {
  max-width: 900px;
}

@media (max-width: 920px) {
  .app { grid-template-columns: 1fr; }
  .sidebar {
    position: fixed;
    inset: 0 auto 0 0;
    width: 320px;
    z-index: 100;
    transform: translateX(-100%);
    transition: transform var(--t-base);
    box-shadow: 8px 0 32px rgba(0,0,0,0.15);
  }
  .sidebar.open { transform: translateX(0); }
  .reader { padding: var(--s-6) var(--s-4); padding-top: var(--s-7); }
  .mobile-bar { display: flex !important; }
}

.mobile-bar {
  display: none;
  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--paper);
  border-bottom: 1px solid var(--border-soft);
  padding: var(--s-3) var(--s-4);
  align-items: center;
  gap: var(--s-3);
}
.mobile-bar button {
  background: transparent;
  border: 1px solid var(--border);
  border-radius: var(--r-sm);
  padding: 6px 10px;
  font-family: var(--font-mono);
  font-size: 0.78rem;
  letter-spacing: 0.08em;
  color: var(--ink);
  cursor: pointer;
  text-transform: uppercase;
}

/* ============================================
   Sidebar — table of contents
   ============================================ */
.toc-brand {
  margin-bottom: var(--s-6);
  padding-bottom: var(--s-5);
  border-bottom: 1px solid var(--border-soft);
}
.toc-brand-mark {
  display: inline-flex;
  align-items: baseline;
  gap: 6px;
  font-family: var(--font-display);
  font-style: italic;
  font-size: 1.5rem;
  color: var(--ink);
  text-decoration: none;
  margin-bottom: 4px;
}
.toc-brand-mark .dot { color: var(--highlight); font-style: normal; }
.toc-brand-sub {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--ink-3);
}

.toc-book {
  margin-bottom: var(--s-5);
}
.toc-book-head {
  display: flex;
  align-items: baseline;
  gap: var(--s-2);
  margin-bottom: var(--s-3);
  padding-bottom: 6px;
  border-bottom: 1px dashed var(--border-soft);
}
.toc-book-num {
  font-family: var(--font-display);
  font-style: italic;
  font-size: 1.1rem;
  color: var(--highlight);
  font-weight: 500;
}
.toc-book-title {
  font-family: var(--font-body);
  font-size: 0.82rem;
  font-weight: 600;
  letter-spacing: 0.02em;
  color: var(--ink-2);
  text-transform: uppercase;
}

.toc-list { list-style: none; padding: 0; margin: 0; display: flex; flex-direction: column; }
.toc-item {
  margin: 0;
  display: block;
}
.toc-link {
  display: grid;
  grid-template-columns: 28px 1fr;
  gap: var(--s-2);
  align-items: baseline;
  padding: 7px 8px 7px 4px;
  border-radius: var(--r-sm);
  text-decoration: none;
  color: var(--ink-2);
  font-size: 0.92rem;
  line-height: 1.3;
  transition: background var(--t-fast), color var(--t-fast);
  border-left: 2px solid transparent;
}
.toc-link:hover { background: rgba(31, 78, 95, 0.08); color: var(--ink); }
.toc-link.active {
  background: var(--accent-soft);
  color: var(--ink);
  border-left-color: var(--accent);
  font-weight: 500;
}
.toc-link.active .toc-num { color: var(--accent); }
.toc-num {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  color: var(--ink-3);
  letter-spacing: 0.06em;
  padding-top: 2px;
}

.toc-special {
  font-family: var(--font-display);
  font-style: italic;
  color: var(--ink-2);
}

/* ============================================
   Chapter header
   ============================================ */
.chapter-header {
  margin-bottom: var(--s-8);
  padding-bottom: var(--s-6);
  border-bottom: 1px solid var(--border-soft);
}
.chapter-meta {
  display: flex;
  align-items: baseline;
  gap: var(--s-4);
  margin-bottom: var(--s-4);
  flex-wrap: wrap;
}
.chapter-meta .eyebrow + .eyebrow::before {
  content: '·';
  margin-right: var(--s-2);
  color: var(--ink-3);
}
.chapter-title {
  font-family: var(--font-display);
  font-size: clamp(2.6rem, 5vw, 4rem);
  line-height: 1.02;
  font-weight: 500;
  letter-spacing: -0.03em;
  margin-bottom: var(--s-4);
}
.chapter-title em { font-style: italic; }
.chapter-dek {
  font-family: var(--font-display);
  font-style: italic;
  font-weight: 400;
  font-size: 1.4rem;
  line-height: 1.4;
  color: var(--ink-2);
  max-width: 38ch;
}

/* ============================================
   Code blocks
   ============================================ */
.codeblock {
  background: var(--code-bg);
  color: var(--code-fg);
  border-radius: var(--r-md);
  margin: var(--s-5) 0;
  overflow: hidden;
  border: 1px solid rgba(255,255,255,0.05);
  position: relative;
}
.codeblock-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 14px;
  background: rgba(255,255,255,0.04);
  border-bottom: 1px solid rgba(255,255,255,0.06);
  font-family: var(--font-mono);
  font-size: 0.72rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--code-comment);
}
.codeblock-lang { color: var(--code-keyword); font-weight: 500; }
.codeblock-copy {
  background: transparent;
  border: 1px solid rgba(255,255,255,0.1);
  color: var(--code-fg);
  font-family: var(--font-mono);
  font-size: 0.68rem;
  padding: 3px 8px;
  border-radius: var(--r-sm);
  cursor: pointer;
  letter-spacing: 0.08em;
  transition: all var(--t-fast);
}
.codeblock-copy:hover { background: rgba(255,255,255,0.06); }
.codeblock pre {
  margin: 0;
  padding: 16px 18px;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.92rem;
  line-height: 1.6;
  font-feature-settings: 'liga' off;
  tab-size: 4;
}
.codeblock pre code {
  background: transparent;
  border: 0;
  padding: 0;
  color: inherit;
  font-size: inherit;
}

/* token classes */
.tok-comment { color: var(--code-comment); font-style: italic; }
.tok-keyword { color: var(--code-keyword); }
.tok-string { color: var(--code-string); }
.tok-number { color: var(--code-number); }
.tok-builtin { color: var(--code-builtin); }
.tok-func { color: var(--code-func); }
.tok-op { color: var(--code-op); }
.tok-prompt { color: var(--code-prompt); user-select: none; }
.tok-output { color: var(--code-comment); }

/* ============================================
   Callouts
   ============================================ */
.callout {
  margin: var(--s-5) 0;
  padding: var(--s-4) var(--s-5);
  border-radius: var(--r-md);
  border-left: 3px solid var(--accent);
  background: var(--accent-soft);
  position: relative;
}
.callout-head {
  display: flex;
  align-items: center;
  gap: var(--s-2);
  margin-bottom: var(--s-2);
  font-family: var(--font-mono);
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.12em;
  color: var(--accent);
  font-weight: 600;
}
.callout-head .glyph {
  font-family: var(--font-display);
  font-style: italic;
  font-size: 1.1rem;
  font-weight: 600;
  width: 22px; height: 22px;
  display: inline-flex; align-items: center; justify-content: center;
  border-radius: 50%;
  background: var(--accent);
  color: var(--paper);
  letter-spacing: 0;
}
.callout p:last-child { margin-bottom: 0; }
.callout code { background: rgba(0,0,0,0.06); border-color: rgba(0,0,0,0.1); }

.callout-warn { background: var(--warn-soft); border-left-color: var(--warn); }
.callout-warn .callout-head { color: var(--warn); }
.callout-warn .callout-head .glyph { background: var(--warn); }

.callout-tip { background: var(--highlight-soft); border-left-color: var(--highlight); }
.callout-tip .callout-head { color: var(--highlight); }
.callout-tip .callout-head .glyph { background: var(--highlight); }

.callout-success { background: var(--ok-soft); border-left-color: var(--ok); }
.callout-success .callout-head { color: var(--ok); }
.callout-success .callout-head .glyph { background: var(--ok); }

/* ============================================
   Quiz
   ============================================ */
.quiz {
  margin: var(--s-6) 0;
  padding: var(--s-5);
  background: var(--paper-2);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
}
.quiz-tag {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-family: var(--font-mono);
  font-size: 0.7rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--amber);
  font-weight: 600;
  margin-bottom: var(--s-3);
}
.quiz-tag::before {
  content: '◆';
  font-size: 0.7rem;
}
.quiz-q {
  font-family: var(--font-display);
  font-size: 1.25rem;
  line-height: 1.35;
  font-weight: 500;
  color: var(--ink);
  margin-bottom: var(--s-4);
}
.quiz-options {
  display: flex;
  flex-direction: column;
  gap: 8px;
  list-style: none;
  padding: 0;
  margin: 0;
}
.quiz-opt {
  display: grid;
  grid-template-columns: 28px 1fr 24px;
  gap: var(--s-3);
  align-items: center;
  padding: 12px 14px;
  background: var(--paper);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-sm);
  cursor: pointer;
  font-family: var(--font-body);
  font-size: 0.95rem;
  color: var(--ink-2);
  text-align: left;
  transition: all var(--t-fast);
  font-weight: 400;
}
.quiz-opt:hover {
  border-color: var(--border);
  background: var(--paper-2);
  color: var(--ink);
}
.quiz-opt-letter {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  font-weight: 600;
  color: var(--ink-3);
  background: var(--paper-2);
  width: 24px; height: 24px;
  display: inline-flex; align-items: center; justify-content: center;
  border-radius: var(--r-sm);
}
.quiz-opt-mark {
  font-family: var(--font-mono);
  font-size: 1rem;
  text-align: center;
}
.quiz-opt.correct {
  background: var(--ok-soft);
  border-color: var(--ok);
  color: var(--ink);
}
.quiz-opt.correct .quiz-opt-letter { background: var(--ok); color: var(--paper); }
.quiz-opt.incorrect {
  background: var(--warn-soft);
  border-color: var(--warn);
  color: var(--ink);
}
.quiz-opt.incorrect .quiz-opt-letter { background: var(--warn); color: var(--paper); }
.quiz-opt.disabled { cursor: default; }
.quiz-feedback {
  margin-top: var(--s-3);
  padding: var(--s-3) var(--s-4);
  border-radius: var(--r-sm);
  font-size: 0.95rem;
  line-height: 1.5;
}
.quiz-feedback.ok { background: var(--ok-soft); color: var(--ink); border-left: 3px solid var(--ok); }
.quiz-feedback.err { background: var(--warn-soft); color: var(--ink); border-left: 3px solid var(--warn); }
.quiz-feedback strong { color: inherit; }

/* ============================================
   Exercise
   ============================================ */
.exercise {
  margin: var(--s-7) 0;
  padding: var(--s-6);
  background: var(--paper-2);
  border: 1px solid var(--border);
  border-radius: var(--r-lg);
  position: relative;
}
.exercise::before {
  content: '';
  position: absolute;
  top: -1px; left: -1px;
  width: 60px; height: 60px;
  border-top: 2px solid var(--highlight);
  border-left: 2px solid var(--highlight);
  border-radius: var(--r-lg) 0 0 0;
  pointer-events: none;
}
.exercise-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  margin-bottom: var(--s-3);
  flex-wrap: wrap;
  gap: var(--s-3);
}
.exercise-tag {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--highlight);
  font-weight: 600;
}
.exercise-diff {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  letter-spacing: 0.1em;
  color: var(--ink-3);
}
.exercise-title {
  font-family: var(--font-display);
  font-size: 1.55rem;
  font-weight: 500;
  line-height: 1.2;
  margin-bottom: var(--s-3);
  letter-spacing: -0.015em;
}
.exercise-body {
  font-size: 1rem;
  color: var(--ink-2);
  margin-bottom: var(--s-4);
}
.exercise-body p:last-child { margin-bottom: 0; }

.runner {
  display: flex;
  flex-direction: column;
  gap: var(--s-3);
  margin-top: var(--s-4);
}
.runner-editor {
  background: var(--code-bg);
  color: var(--code-fg);
  border-radius: var(--r-md);
  overflow: hidden;
}
.runner-editor-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 14px;
  background: rgba(255,255,255,0.04);
  font-family: var(--font-mono);
  font-size: 0.7rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--code-comment);
  border-bottom: 1px solid rgba(255,255,255,0.06);
}
.runner-editor textarea {
  width: 100%;
  background: transparent;
  border: 0;
  outline: 0;
  font-family: var(--font-mono);
  font-size: 0.92rem;
  line-height: 1.6;
  color: var(--code-fg);
  padding: 14px 18px;
  resize: vertical;
  min-height: 110px;
  font-feature-settings: 'liga' off;
  tab-size: 4;
}
.runner-actions {
  display: flex;
  gap: var(--s-2);
  flex-wrap: wrap;
  align-items: center;
}
.btn {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-family: var(--font-body);
  font-size: 0.92rem;
  font-weight: 500;
  padding: 9px 16px;
  border-radius: var(--r-sm);
  cursor: pointer;
  border: 1px solid transparent;
  transition: all var(--t-fast);
  text-decoration: none;
}
.btn-primary { background: var(--accent); color: var(--paper); }
.btn-primary:hover { background: var(--accent-hover); color: var(--paper); }
.btn-ghost { background: transparent; border-color: var(--border); color: var(--ink); }
.btn-ghost:hover { border-color: var(--ink); background: var(--paper-2); }
.btn-link {
  background: transparent;
  border: 0;
  color: var(--accent);
  text-decoration: underline;
  text-underline-offset: 3px;
  padding: 6px 0;
  font-size: 0.9rem;
}
.btn-link:hover { color: var(--highlight); }

.runner-output {
  background: var(--paper);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
  padding: 12px 16px;
  font-family: var(--font-mono);
  font-size: 0.88rem;
  line-height: 1.55;
  color: var(--ink);
  white-space: pre-wrap;
  word-break: break-word;
  min-height: 44px;
}
.runner-output.err { background: var(--warn-soft); border-color: var(--warn); color: var(--ink); }
.runner-output.empty { color: var(--ink-3); font-style: italic; font-family: var(--font-display); }
.runner-status {
  font-family: var(--font-mono);
  font-size: 0.72rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
}
.runner-status.loading { color: var(--amber); }
.runner-status.ready { color: var(--ok); }
.runner-status.err { color: var(--warn); }

.solution {
  margin-top: var(--s-4);
  border-top: 1px dashed var(--border);
  padding-top: var(--s-4);
}
.solution summary {
  font-family: var(--font-mono);
  font-size: 0.78rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
  cursor: pointer;
  user-select: none;
  padding: 6px 0;
}
.solution summary:hover { color: var(--accent); }
.solution[open] summary { color: var(--accent); margin-bottom: var(--s-3); }
.solution-body { font-size: 0.95rem; color: var(--ink-2); }
.solution-body p:last-child { margin-bottom: 0; }

/* ============================================
   Pull quote
   ============================================ */
.pullquote {
  margin: var(--s-7) 0;
  padding: var(--s-5) var(--s-6);
  border-left: 3px solid var(--highlight);
  font-family: var(--font-display);
  font-style: italic;
  font-size: 1.5rem;
  line-height: 1.35;
  color: var(--ink);
  font-weight: 400;
  letter-spacing: -0.01em;
}

/* ============================================
   Chapter navigation footer
   ============================================ */
.chapter-nav {
  margin-top: var(--s-9);
  padding-top: var(--s-6);
  border-top: 1px solid var(--border-soft);
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--s-4);
}
.chapter-nav-link {
  display: block;
  padding: var(--s-4) var(--s-5);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-md);
  text-decoration: none;
  color: var(--ink-2);
  transition: all var(--t-fast);
}
.chapter-nav-link:hover {
  border-color: var(--border);
  background: var(--paper-2);
  color: var(--ink);
}
.chapter-nav-link.next { text-align: right; }
.chapter-nav-link .dir {
  font-family: var(--font-mono);
  font-size: 0.7rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--ink-3);
}
.chapter-nav-link .label {
  font-family: var(--font-display);
  font-size: 1.15rem;
  margin-top: 4px;
  display: block;
  color: var(--ink);
  font-weight: 500;
  line-height: 1.25;
}
.chapter-nav-link.disabled {
  opacity: 0.35;
  pointer-events: none;
}

/* ============================================
   Cover & intro pages
   ============================================ */
.cover {
  min-height: calc(100vh - var(--s-8) * 2);
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  width: 100%;
  max-width: 1100px;
  margin: 0 auto;
  position: relative;
}
.cover-grid {
  display: grid;
  grid-template-columns: 1fr 380px;
  gap: var(--s-8);
  align-items: end;
  flex: 1;
  padding: var(--s-7) 0;
}
@media (max-width: 800px) {
  .cover-grid { grid-template-columns: 1fr; }
}

/* ============================================
   Progress dots, misc
   ============================================ */
.progress {
  position: fixed;
  bottom: var(--s-4);
  right: var(--s-4);
  z-index: 20;
  background: var(--paper-2);
  border: 1px solid var(--border-soft);
  border-radius: var(--r-pill, 999px);
  padding: 6px 12px;
  font-family: var(--font-mono);
  font-size: 0.72rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-3);
}

/* hide scrollbars softly */
.sidebar::-webkit-scrollbar { width: 6px; }
.sidebar::-webkit-scrollbar-thumb { background: var(--border); border-radius: 999px; }
.sidebar::-webkit-scrollbar-track { background: transparent; }

/* Smooth focus rings */
:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 2px;
}