@import url('https://fonts.googleapis.com/css2?family=Source+Serif+4:ital,opsz,wght@0,8..60,400;0,8..60,500;0,8..60,600;1,8..60,400&family=IBM+Plex+Sans:wght@400;500;600;700&family=IBM+Plex+Mono:wght@400;500&display=swap');

/* =================================================================
   book4matter — web stylesheet
   Drop-in CSS for the generated HTML. Link it AFTER the generator's
   own <style> blocks so these rules win on cascade order, e.g.:

       <link rel="stylesheet" href="book4matter-web.css" />

   Theme: clean docs, sticky sidebar table of contents, serif reading
   body (Source Serif 4) with sans headings (IBM Plex Sans) and a
   mono for code (IBM Plex Mono). Warm-neutral light palette, one
   muted-blue accent, contrast-checked (body text ~16:1, muted ~7:1,
   links ~7:1).
   ================================================================= */

:root {
  --side-w: 300px;        /* sidebar width            */
  --head-h: 86px;         /* fixed title bar height   */
  --measure: 720px;       /* reading column max-width */

  --bg:        #fbfaf8;   /* page background          */
  --bg-side:   #f3f1ec;   /* sidebar background       */
  --ink:       #1c1b19;   /* primary text             */
  --ink-2:     #57534c;   /* muted text / captions    */
  --rule:      #e4e0d8;   /* hairlines & borders      */
  --rule-2:    #d8d3c9;   /* stronger borders         */
  --accent:    #2f5d8a;   /* links, active nav, marks */
  --accent-bg: #e9eef5;   /* tint behind active nav   */
  --code-bg:   #f1ede6;   /* inline code background   */
  --block-bg:  #f6f3ed;   /* code block background    */

  /* Kept in sync with the default blockquote map in book_style.yaml. The
     generated per-book CSS overrides these when a book supplies the map. */
  --blockquote-width:      80%;
  --blockquote-rule-width: 50%;
  --blockquote-spacing:    1.25em;
  --blockquote-text-align:  center;
  --blockquote-font-style: italic;

  --serif: 'Source Serif 4', Georgia, 'Times New Roman', serif;
  --sans:  'IBM Plex Sans', system-ui, 'Segoe UI', Helvetica, sans-serif;
  --mono:  'IBM Plex Mono', ui-monospace, 'SF Mono', Menlo, monospace;
}

/* ---- Base / reading column ------------------------------------ */

html { scroll-behavior: smooth; }

body {
  font-family: var(--serif);
  font-size: 19px;
  line-height: 1.72;
  color: var(--ink);
  background: var(--bg);
  margin: 0;
  padding-left: var(--side-w);
  text-align: left;          /* override justify */
  -webkit-hyphens: none;
  hyphens: none;
  -webkit-font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}

/* Each top-level content block is the centered reading column. */
body > section,
body > .footnotes {
  max-width: var(--measure);
  margin: 0 auto;
  padding: 8px 56px 96px;
  box-sizing: content-box;
}
body > section:first-of-type { padding-top: 64px; }

p {
  margin: 0 0 1.05em;
  text-indent: 0;            /* web spacing, not print indent */
}

/* Comfortable measure for prose; keep code/figures full width. */
p, li, blockquote { max-width: 68ch; }

a {
  color: var(--accent);
  text-decoration-color: color-mix(in srgb, var(--accent) 35%, transparent);
  text-underline-offset: 2px;
}
a:hover { text-decoration-color: var(--accent); }

strong { font-weight: 600; }
em { font-style: italic; }

::selection { background: var(--accent-bg); }

/* ---- Headings -------------------------------------------------- */

h1, h2, h3, h4, h5, h6 {
  font-family: var(--sans);
  color: var(--ink);
  text-transform: none;       /* drop the print uppercase */
  letter-spacing: -0.01em;
  line-height: 1.22;
  text-align: left;
  font-weight: 600;
  text-wrap: balance;
  -webkit-hyphens: none;
  hyphens: none;
  page-break-after: auto;
  break-after: auto;
  scroll-margin-top: 1.5rem;
}

/* h1 = a Part divider */
h1 {
  font-size: 2.1em;
  font-weight: 700;
  margin: 0 0 0.7em;
  padding-top: 0.4em;
  text-align: left;
  page-break-after: auto;
  break-after: auto;
}
/* h2 = a Chapter / top section */
h2 {
  font-size: 1.62em;
  font-weight: 700;
  margin: 1.9em 0 0.6em;
  padding-top: 1.4em;
  border-top: 1px solid var(--rule);
}
body > section:first-of-type > h2,        /* Introduction: no top rule */
section.level1 > h2:first-child {
  border-top: none;
  padding-top: 0;
}
section.level1 > h2 { margin-top: 1.2em; }

h3 {
  font-size: 1.18em;
  font-weight: 600;
  margin: 2em 0 0.5em;
  letter-spacing: 0;
}
h4, h5, h6 {
  font-size: 1em;
  font-weight: 600;
  margin: 1.6em 0 0.4em;
  letter-spacing: 0;
  color: var(--ink-2);
}

/* Part / Chapter eyebrows printed inside the headings */
h1 .part-label,
h2 .chapter-label {
  display: block;
  font-family: var(--sans);
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.13em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 0.7em;
}

/* Heading directly followed by text gets no extra gap */
h1 + p, h2 + p, h3 + p, h4 + p, h5 + p, h6 + p { margin-top: 0; }

/* ---- Title bar (header) --------------------------------------- */

#title-block-header {
  position: fixed;
  top: 0;
  left: 0;
  width: var(--side-w);
  height: var(--head-h);
  box-sizing: border-box;
  display: flex;
  align-items: center;
  padding: 0 28px;
  margin: 0;
  background: var(--bg-side);
  border-right: 1px solid var(--rule-2);
  border-bottom: 1px solid var(--rule);
  z-index: 20;
}
#title-block-header .title,
h1.title {
  font-family: var(--sans);
  font-size: 1.32rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  text-transform: none;
  color: var(--ink);
  margin: 0;
  padding: 0;
  line-height: 1.1;
  text-align: left;
  page-break-after: auto;
  break-after: auto;
}
#title-block-header .title::before {
  content: "";
  display: inline-block;
  width: 9px;
  height: 9px;
  margin-right: 11px;
  border-radius: 2px;
  background: var(--accent);
  transform: translateY(-1px);
}

/* ---- Sidebar table of contents -------------------------------- */

#TOC {
  position: fixed;
  top: var(--head-h);
  left: 0;
  width: var(--side-w);
  height: calc(100vh - var(--head-h));
  box-sizing: border-box;
  overflow-y: auto;
  overscroll-behavior: contain;
  padding: 26px 22px 40px;
  background: var(--bg-side);
  border-right: 1px solid var(--rule-2);
  z-index: 10;
}
#TOC::before {
  content: "Contents";
  display: block;
  font-family: var(--sans);
  font-size: 0.7rem;
  font-weight: 600;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--ink-2);
  padding: 0 10px;
  margin-bottom: 12px;
}

#TOC ul {
  list-style: none;
  margin: 0;
  padding: 0;
}
#TOC li { margin: 1px 0; max-width: none; }

#TOC a {
  display: block;
  font-family: var(--sans);
  font-size: 0.92rem;
  line-height: 1.32;
  color: var(--ink);
  text-decoration: none;
  padding: 7px 10px;
  border-radius: 7px;
  transition: background 0.12s ease, color 0.12s ease;
}
#TOC a:hover { background: color-mix(in srgb, var(--accent) 9%, transparent); }
#TOC a:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 1px;
}

/* Part / chapter eyebrow inside TOC links */
#TOC .part-label,
#TOC .chapter-label {
  display: block;
  font-size: 0.62rem;
  font-weight: 600;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--ink-2);
  margin-bottom: 2px;
}

/* Top-level links (Introduction, Parts) read as section heads */
#TOC > ul > li > a {
  font-weight: 600;
  margin-top: 6px;
}

/* Nested chapter list, with a guide rule */
#TOC ul ul {
  margin: 2px 0 8px;
  padding-left: 11px;
  border-left: 1px solid var(--rule-2);
}
#TOC ul ul a { font-weight: 400; color: var(--ink-2); }
#TOC ul ul a:hover { color: var(--ink); }
#TOC ul ul .chapter-label { margin-bottom: 1px; }

/* ---- Code ------------------------------------------------------ */

code, kbd, samp {
  font-family: var(--mono);
  font-size: 0.86em;
}
:not(pre) > code {
  background: var(--code-bg);
  color: var(--ink);
  padding: 0.12em 0.4em;
  border-radius: 5px;
  border: 1px solid var(--rule);
  white-space: nowrap;
}
pre {
  font-family: var(--mono);
  background: var(--block-bg);
  border: 1px solid var(--rule-2);
  border-radius: 10px;
  padding: 18px 20px;
  margin: 1.5em 0;
  line-height: 1.6;
  white-space: pre-wrap;
  overflow-x: auto;
  max-width: none;
}
pre code {
  font-size: 0.82em;
  background: none;
  border: none;
  padding: 0;
  white-space: pre-wrap;
}

/* ---- Blockquote ------------------------------------------------ */

blockquote {
  width: var(--blockquote-width);
  max-width: none;
  margin: var(--blockquote-spacing) auto;
  padding: 0;
  text-align: var(--blockquote-text-align);
  color: var(--ink-2);
  font-style: var(--blockquote-font-style);
}
blockquote::before,
blockquote::after {
  content: "";
  display: block;
  width: var(--blockquote-rule-width);
  margin-left: auto;
  margin-right: auto;
  border-top: 1px solid currentColor;
}
blockquote::before { margin-bottom: 0.8em; }
blockquote::after  { margin-top: 0.8em; }
blockquote p {
  max-width: none;
  margin-bottom: 0.6em;
  text-align: inherit;
}
blockquote p:last-child { margin-bottom: 0; }

/* ---- Lists ----------------------------------------------------- */

ul, ol { margin: 0 0 1.05em 1.4em; padding: 0; }
li { margin: 0.3em 0; }
li > ul, li > ol { margin-bottom: 0.3em; }

/* ---- Figures & images ----------------------------------------- */

figure { margin: 1.8em 0; text-align: center; }
figure img, img.center { max-width: 100%; height: auto; border-radius: 6px; }
figcaption {
  font-family: var(--sans);
  font-size: 0.82em;
  font-style: normal;
  color: var(--ink-2);
  margin-top: 0.6em;
}

/* ---- Thematic break (* * *) ----------------------------------- */

hr {
  border: none;
  text-align: center;
  margin: 2.4em 0;
}
hr::after {
  content: "\2217\00a0\00a0\2217\00a0\00a0\2217";  /* ∗  ∗  ∗ */
  font-family: var(--serif);
  color: var(--ink-2);
  letter-spacing: 0.2em;
  font-size: 1em;
}

/* ---- Footnotes ------------------------------------------------- */

a.footnote-ref {
  vertical-align: super;
  font-size: 0.72em;
  line-height: 0;
  text-decoration: none;
  color: var(--accent);
  font-family: var(--sans);
  padding: 0 0.1em;
}
.footnotes {
  margin-top: 3em;
  font-size: 0.92em;
  color: var(--ink-2);
}
.footnotes hr {
  border-top: 1px solid var(--rule-2);
  width: 56px;
  margin: 1.5em 0;
}
.footnotes hr::after { content: none; }

/* ---- :target highlight (pure-CSS nav feedback) ---------------- */

section:target > h1,
section:target > h2,
section:target > h3 {
  position: relative;
}
section:target > h1::before,
section:target > h2::before,
section:target > h3::before {
  content: "";
  position: absolute;
  left: -22px;
  top: 0.15em;
  bottom: 0.15em;
  width: 3px;
  border-radius: 3px;
  background: var(--accent);
}

/* ================================================================
   Responsive: collapse the fixed sidebar on narrow screens.
   The TOC becomes a normal block at the top of the page.
   ================================================================ */

@media (max-width: 900px) {
  body { padding-left: 0; }

  #title-block-header {
    position: static;
    width: auto;
    height: auto;
    padding: 20px 24px;
    border-right: none;
  }
  #TOC {
    position: static;
    width: auto;
    height: auto;
    max-height: none;
    overflow: visible;
    border-right: none;
    border-bottom: 1px solid var(--rule);
    padding: 20px 24px 26px;
  }
  #TOC ul ul { border-left-color: var(--rule); }

  body > section,
  body > .footnotes {
    padding-left: 24px;
    padding-right: 24px;
  }
  body > section:first-of-type { padding-top: 34px; }
}

@media (max-width: 560px) {
  body { font-size: 18px; }
}