:root{
  --bg:#f7f6f2; --surface:#fff; --ink:#1d2433; --muted:#5d6676; --line:#e2dfd6; --soft:#f1efe9;
  --accent:#0f766e; --accent-bg:#e8f5f2;
  --warn:#a14a07; --warn-bg:#fff3e2;
  --good:#15803d; --okay:#a16207; --bad:#b91c1c;
}
@media (prefers-color-scheme: dark){
  :root{
    --bg:#121419; --surface:#1a1d24; --ink:#e5e8ef; --muted:#9aa3b3; --line:#2c313c; --soft:#21252d;
    --accent:#4fd1bf; --accent-bg:#14302c;
    --warn:#f6b250; --warn-bg:#30230f;
    --good:#4ade80; --okay:#facc15; --bad:#f87171;
  }
}

*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0 auto;max-width:1240px;display:grid;grid-template-columns:270px minmax(0,1fr);align-items:start;background:var(--bg);color:var(--ink);font:16px/1.6 system-ui,-apple-system,"Segoe UI",Roboto,Ubuntu,sans-serif}
a{color:inherit}
main{padding:28px 40px 80px;min-width:0}
.katex{text-transform:none;letter-spacing:normal;position:relative} /* relative: keeps KaTeX's hidden MathML copy inside .tablewrap, or a wide table makes the page scroll sideways */
.katex-display{overflow-x:auto;overflow-y:hidden;margin:.4em 0;padding:2px 0}
.small{font-size:14px;color:var(--muted)}

/* contents sidebar (built by study.js) */
#toc{position:sticky;top:0;max-height:100vh;overflow:auto;margin:0;padding:22px 14px 40px 20px;border:0;border-radius:0;border-right:1px solid var(--line);font-size:14px}
#toc > summary{list-style:none;cursor:pointer;font-weight:700;font-size:15px;margin-bottom:8px}
#toc > summary::-webkit-details-marker{display:none}
#toc > summary small{display:block;font-weight:400;font-size:12.5px;color:var(--muted)}
#toc ol{list-style:none;margin:0;padding:0}
#toc .toc-part{margin:16px 0 4px;font-size:11.5px;letter-spacing:.06em;text-transform:uppercase;color:var(--muted);font-weight:700}
#toc a{display:flex;gap:8px;padding:3px 8px;border-radius:6px;text-decoration:none;line-height:1.35}
#toc a:hover{background:var(--soft)}
#toc .toc-sub a{padding-left:24px;font-size:13px;color:var(--muted)}
#toc .n{color:var(--muted);min-width:24px;font-variant-numeric:tabular-nums}

/* page intro */
.hero h1{font-size:30px;line-height:1.2;margin:0 0 6px}
.hero p{color:var(--muted);margin:0 0 18px;max-width:70ch}
.card{background:var(--surface);border:1px solid var(--line);border-radius:12px;padding:18px 20px;margin:0 0 16px}
.card > :first-child{margin-top:0}.card > :last-child{margin-bottom:0}
.road{display:flex;flex-wrap:wrap;gap:6px;align-items:center}
.road a{border:1px solid var(--line);background:var(--soft);padding:2px 10px;border-radius:999px;text-decoration:none;font-size:14px}

/* parts */
.part{margin:48px 0 14px;padding-top:6px;border-top:2px solid var(--ink)}
.part h1{font-size:12.5px;letter-spacing:.08em;text-transform:uppercase;margin:8px 0 0;color:var(--muted)}
.part p{margin:2px 0 0;font-size:20px;font-weight:700}
.part p.note{font-size:14px;font-weight:400;color:var(--muted)}

/* concept card and subcards */
.concept{background:var(--surface);border:1px solid var(--line);border-radius:14px;padding:22px 24px 18px;margin:0 0 22px}
.sub{border-left:3px solid var(--line);padding-left:16px;margin:22px 0 8px}
.c-head{display:flex;flex-wrap:wrap;align-items:baseline;justify-content:space-between;gap:8px;margin-bottom:6px}
.c-head > *{margin:0;line-height:1.25}
.concept > .c-head h2{font-size:22px}
.sub > .c-head > :is(h3,h4){font-size:17px}
.num{color:var(--muted);font-weight:500;margin-right:8px}
.src{font-size:12.5px;color:var(--muted);white-space:nowrap}
[data-added] > .c-head .src{color:var(--accent);font-weight:600}
.links{font-size:13.5px;color:var(--muted);display:flex;flex-wrap:wrap;gap:2px 14px;margin:0 0 10px}
a.chip{display:inline-block;text-decoration:none;color:var(--ink);background:var(--soft);border:1px solid var(--line);border-radius:999px;padding:0 9px;margin:2px 3px 0 0;font-size:13px}
a.chip:hover{border-color:var(--muted)}
.label{font-size:12.5px;margin:18px 0 4px;text-transform:uppercase;letter-spacing:.06em;color:var(--muted);font-weight:700}

/* boxes: plain = the summary, added = extra explanation, warn = mistakes, mine = your notes, slide = slide content */
.plain,.added,.warn,.mine,.slide{margin:12px 0;padding:10px 14px;border-radius:8px;border-left:4px solid}
.plain{background:var(--accent-bg);border-color:var(--accent);font-size:16.5px}
.added{border-color:var(--accent)}
.warn{background:var(--warn-bg);border-color:var(--warn)}
.mine{border:1px dashed var(--muted)}
.slide{border-color:var(--line);border-radius:0;padding:2px 0 2px 14px}
:is(.plain,.added,.warn,.mine,.slide) > :first-child{margin-top:0}
:is(.plain,.added,.warn,.mine,.slide) > :last-child{margin-bottom:0}
:is(.plain,.added,.warn,.mine,.slide)::before{display:block;font-size:11.5px;font-weight:700;letter-spacing:.06em;text-transform:uppercase;color:var(--muted)}
.plain::before{content:"In plain words";color:var(--accent)}
.added::before{content:"Added";color:var(--accent)}
.warn::before{content:"Watch out";color:var(--warn)}
.mine::before{content:"My notes"}
.slide::before{content:"From the slides"}
[data-label]::before{content:attr(data-label)}

dl.sym{display:grid;grid-template-columns:max-content 1fr;gap:2px 14px;font-size:14.5px;margin:8px 0;padding:8px 12px;background:var(--soft);border-radius:8px}
dl.sym dt{font-weight:600}
dl.sym dd{margin:0}

/* one step (one equation) per item */
ol.steps{counter-reset:s;list-style:none;padding:0;margin:8px 0}
ol.steps > li{counter-increment:s;position:relative;padding:4px 0 4px 36px}
ol.steps .katex-display{margin:.15em 0}
ol.steps .katex-display > .katex{text-align:left}
ol.steps > li::before{content:counter(s);position:absolute;left:0;top:5px;width:24px;height:24px;border-radius:50%;background:var(--soft);border:1px solid var(--line);font-size:12.5px;font-weight:700;display:flex;align-items:center;justify-content:center}

/* dropdowns: worked examples, practice, hidden answers */
details{border:1px solid var(--line);border-radius:10px;margin:10px 0}
details > summary{cursor:pointer;padding:9px 14px;font-weight:600;font-size:14.5px}
details > :not(summary){margin-left:16px;margin-right:16px}
details[open] > :last-child{margin-bottom:12px}
details.ans{border-style:dashed;background:var(--soft)}
details.ans > summary{font-size:13.5px;color:var(--muted)}
.mark{margin-left:10px;font-size:12.5px;white-space:nowrap}
.mark::before{content:"● "}
.got{color:var(--good)} .shaky{color:var(--okay)} .missed{color:var(--bad)}
.rate{font-size:14px;color:var(--muted)}
.rate button{font:600 13px system-ui,sans-serif;background:var(--surface);border:1px solid currentColor;border-radius:999px;padding:2px 12px;margin:2px 0 0 6px;cursor:pointer}
.tag{font-size:12px;font-weight:700;letter-spacing:.04em;text-transform:uppercase;color:var(--muted);margin-right:8px}

table{border-collapse:collapse;width:100%;font-size:14px;margin:8px 0}
th,td{border-bottom:1px solid var(--line);padding:6px 8px;text-align:left;vertical-align:top}
th{color:var(--muted);font-size:12.5px;text-transform:uppercase;letter-spacing:.04em}
.tablewrap,.lecture-map{overflow-x:auto}
figure{margin:12px 0}
figure img{max-width:100%;height:auto;border-radius:8px;border:1px solid var(--line);background:#fff}
figcaption{font-size:13px;color:var(--muted);margin-top:4px}
footer{color:var(--muted);font-size:13.5px;margin-top:40px}

@media (max-width:900px){
  body{display:block}
  #toc{position:static;max-height:none;border-right:0;border-bottom:1px solid var(--line);padding:12px 16px}
  main{padding:20px 16px 60px}
  .concept{padding:18px 16px 14px}
  .sub{padding-left:12px}
}