:root {
  --bg: #fbfaf7;
  --text: #24252a;
  --muted: #6d6a62;
  --rule: #e4e1d8;
  --side-bg: #23262d;
  --side-text: #c9c7c0;
  --side-head: #8d8b84;
  --side-hover: #343842;
  --accent: #2f6fdf;
  --accent-soft: #e8effc;
  --code-bg: #f3f1ea;
  --note-bg: #eef5ff;
  --note-rule: #8fb3ef;
  --warn-bg: #fff4e5;
  --warn-rule: #f0b25c;
  --ex-bg: #effaf2;
  --ex-rule: #67bf86;
  --paper-bg: #f6f0ff;
  --paper-rule: #a98be0;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #1b1d22; --text: #e3e1da; --muted: #a29f96; --rule: #34373f;
    --side-bg: #15171b; --side-hover: #262931; --accent: #79a6ff; --accent-soft: #23304a;
    --code-bg: #24272e; --note-bg: #1f2a3c; --note-rule: #4f74b3; --warn-bg: #352a19; --warn-rule: #b9853c;
    --ex-bg: #1b2f23; --ex-rule: #3f8c5a; --paper-bg: #2a2338; --paper-rule: #7d63b0;
  }
}
:root[data-theme="dark"] {
  --bg: #1b1d22; --text: #e3e1da; --muted: #a29f96; --rule: #34373f;
  --side-bg: #15171b; --side-hover: #262931; --accent: #79a6ff; --accent-soft: #23304a;
  --code-bg: #24272e; --note-bg: #1f2a3c; --note-rule: #4f74b3; --warn-bg: #352a19; --warn-rule: #b9853c;
  --ex-bg: #1b2f23; --ex-rule: #3f8c5a; --paper-bg: #2a2338; --paper-rule: #7d63b0;
}

* { box-sizing: border-box; }
html { scroll-padding-top: 16px; }
body {
  margin: 0; background: var(--bg); color: var(--text);
  font: 17px/1.7 Charter, "Bitstream Charter", "Sitka Text", Cambria, Georgia, serif;
}
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

#sidebar {
  position: fixed; inset: 0 auto 0 0; width: 280px; overflow-y: auto; padding: 18px 0 40px;
  background: var(--side-bg); color: var(--side-text);
  font: 14px/1.4 system-ui, -apple-system, "Segoe UI", sans-serif;
}
#sidebar .brand { display: block; padding: 0 20px 14px; color: #fff; font-weight: 700; font-size: 17px; }
#sidebar .part {
  padding: 14px 20px 4px; color: var(--side-head); text-transform: uppercase; font-size: 11.5px; letter-spacing: 0.08em;
}
#sidebar ul { list-style: none; margin: 0; padding: 0; }
#sidebar li a { display: flex; gap: 10px; padding: 4px 20px; color: var(--side-text); }
#sidebar li a:hover { background: var(--side-hover); text-decoration: none; color: #fff; }
#sidebar li.current a { background: var(--accent); color: #fff; }
#sidebar .num { color: var(--side-head); font-variant-numeric: tabular-nums; min-width: 1.4em; }
#sidebar li.current .num { color: #dfe8ff; }

#menu-toggle {
  display: none; position: fixed; top: 10px; left: 10px; z-index: 20; padding: 6px 10px;
  font: 14px system-ui, sans-serif; border-radius: 4px; border: 1px solid var(--rule); background: var(--bg); color: var(--text);
}

main { margin-left: 280px; padding: 40px 48px 80px; }
article { max-width: 780px; margin: 0 auto; }
article > h1 { font: 700 2.1em/1.2 system-ui, -apple-system, "Segoe UI", sans-serif; margin: 0 0 0.2em; }
article .subtitle { color: var(--muted); margin: 0 0 1.6em; font-style: italic; }
article h2 { font: 700 1.45em/1.3 system-ui, -apple-system, "Segoe UI", sans-serif; margin: 2em 0 0.6em; padding-top: 0.6em; border-top: 1px solid var(--rule); }
article h3 { font: 700 1.15em/1.3 system-ui, -apple-system, "Segoe UI", sans-serif; margin: 1.6em 0 0.5em; }
article h4 { font: 700 1em/1.3 system-ui, -apple-system, "Segoe UI", sans-serif; margin: 1.2em 0 0.4em; }
article p, article li { hyphens: auto; }
article img, article svg { max-width: 100%; }
article figure { margin: 1.5em 0; text-align: center; }
article figcaption { color: var(--muted); font-size: 0.88em; margin-top: 0.4em; }
article table { border-collapse: collapse; margin: 1.2em 0; font-size: 0.92em; width: 100%; }
article th, article td { border: 1px solid var(--rule); padding: 6px 10px; text-align: left; vertical-align: top; }
article th { background: var(--code-bg); font-family: system-ui, sans-serif; }
article blockquote { margin: 1em 0; padding: 0 1em; border-left: 3px solid var(--rule); color: var(--muted); }

code { font: 0.86em/1.5 "JetBrains Mono", "Cascadia Code", Consolas, monospace; background: var(--code-bg); padding: 0.1em 0.3em; border-radius: 3px; }
pre { background: var(--code-bg); padding: 14px 16px; border-radius: 6px; overflow-x: auto; line-height: 1.5; }
pre code { background: none; padding: 0; font-size: 0.84em; }
pre code.hljs { background: none; padding: 0; }

.note, .warning, .exercise, .paper {
  margin: 1.4em 0; padding: 12px 18px; border-left: 4px solid; border-radius: 0 6px 6px 0;
}
.note { background: var(--note-bg); border-color: var(--note-rule); }
.warning { background: var(--warn-bg); border-color: var(--warn-rule); }
.exercise { background: var(--ex-bg); border-color: var(--ex-rule); }
.paper { background: var(--paper-bg); border-color: var(--paper-rule); }
.note > :first-child, .warning > :first-child, .exercise > :first-child, .paper > :first-child { margin-top: 0; }
.note > :last-child, .warning > :last-child, .exercise > :last-child, .paper > :last-child { margin-bottom: 0; }
.exercise h4::before { content: "Your turn: "; color: var(--ex-rule); }
.paper h4::before { content: "Paper corner: "; color: var(--paper-rule); }

details { margin: 0.8em 0; border: 1px solid var(--rule); border-radius: 6px; padding: 6px 14px; background: var(--bg); }
details summary { cursor: pointer; font: 600 0.92em system-ui, sans-serif; color: var(--muted); }
details[open] summary { margin-bottom: 0.4em; }

.demo { margin: 1.6em 0; }
.demo-frame {
  position: relative; border: 1px solid var(--rule); border-radius: 6px; overflow: hidden; background: #fbfaf7;
  display: flex; align-items: center; justify-content: center;
}
.demo-frame iframe { position: absolute; inset: 0; width: 100%; height: 100%; border: 0; }
.demo-start {
  font: 600 15px system-ui, sans-serif; padding: 10px 18px; border-radius: 6px; cursor: pointer;
  border: 1px solid #c9d8f5; background: #e8effc; color: #2f6fdf;
}
.demo-start:hover { background: #dbe7fb; }
.demo-links { display: flex; justify-content: space-between; gap: 12px; font: 13px system-ui, sans-serif; color: var(--muted); margin-top: 6px; }
.demo-links a { margin-left: auto; }

.files { font: 14px system-ui, sans-serif; }
.files a { display: inline-block; margin-right: 14px; }

.pager { display: flex; justify-content: space-between; gap: 16px; margin-top: 3em; padding-top: 1.2em; border-top: 1px solid var(--rule); font: 15px system-ui, sans-serif; }
.pager a.next { margin-left: auto; text-align: right; }

#toc h3 { font: 700 1em system-ui, sans-serif; margin: 1.4em 0 0.3em; color: var(--muted); text-transform: uppercase; letter-spacing: 0.06em; font-size: 0.8em; }
#toc ol { list-style: none; padding: 0; margin: 0; columns: 2; column-gap: 32px; }
#toc li { break-inside: avoid; padding: 2px 0; }
#toc .num { color: var(--muted); font-variant-numeric: tabular-nums; }

footer { margin: 4em auto 0; max-width: 780px; color: var(--muted); font: 13px system-ui, sans-serif; }

.katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; }

@media (max-width: 900px) {
  #sidebar { transform: translateX(-100%); transition: transform 0.2s; z-index: 15; }
  body.menu-open #sidebar { transform: none; }
  #menu-toggle { display: block; }
  main { margin-left: 0; padding: 56px 16px 60px; }
  #toc ol { columns: 1; }
}

/* Colour roles shared with the \now and \nxt maths macros (site.js). */
.now { color: #3d7be0; font-weight: 600; }
.nxt { color: #e07b14; font-weight: 600; }
.check { margin: 1.4em 0; padding: 10px 16px; border: 1px dashed var(--rule); border-radius: 6px; }
.check > :first-child { margin-top: 0; }
.check > :last-child { margin-bottom: 0; }
.check h4::before { content: "Check yourself: "; color: var(--muted); }
.compare td, .compare th { text-align: center; }
.compare td:first-child, .compare th:first-child { text-align: left; }
.deeper { border-left: 4px solid var(--rule); }

/* Puzzles (site.js setupPuzzles) */
.puzzle { margin: 1.6em 0; padding: 12px 18px; border: 1px solid var(--rule); border-left: 4px solid var(--warn-rule); border-radius: 0 6px 6px 0; }
.puzzle > :first-child { margin-top: 0; }
.puzzle h4::before { content: "Puzzle: "; color: var(--warn-rule); }
.puzzle.solved { border-left-color: var(--ex-rule); }
.puzzle.solved h4::after { content: "  \2713 solved"; color: var(--ex-rule); font-weight: 600; font-size: 0.85em; }
.puzzle-form { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; margin: 0.8em 0 0.3em; font: 15px system-ui, sans-serif; }
.puzzle-form input[type=text] { width: 9em; padding: 5px 8px; font: inherit; border: 1px solid var(--rule); border-radius: 4px; background: var(--bg); color: var(--text); }
.puzzle-unit { color: var(--muted); }
.puzzle-choice { margin-right: 10px; cursor: pointer; }
.puzzle-check, .puzzle-reveal { font: 600 14px system-ui, sans-serif; padding: 5px 12px; border-radius: 4px; cursor: pointer; border: 1px solid var(--rule); background: var(--code-bg); color: var(--text); }
.puzzle-feedback { min-height: 1.3em; font: 14px system-ui, sans-serif; color: var(--muted); }
.puzzle-feedback.ok { color: var(--ex-rule); font-weight: 600; }
.puzzle-feedback.no { color: var(--warn-rule); }
.puzzle .worked { margin-top: 0.8em; padding-top: 0.6em; border-top: 1px dashed var(--rule); }
.puzzle .worked::before { content: "Worked solution"; display: block; font: 600 0.85em system-ui, sans-serif; color: var(--muted); margin-bottom: 0.3em; }