/* ==========================================================================
   501 Probability — shared course stylesheet
   Loaded by every page of the course. See chapter-guide.html for rules.
   ========================================================================== */

:root{
  --bg:        #0e1117;
  --bg-soft:   #141a24;
  --panel:     #171f2b;
  --panel-2:   #1b2432;
  --line:      #2a3446;
  --ink:       #e9e4d8;
  --ink-dim:   #a9b0bd;
  --ink-faint: #6b7688;
  --gold:      #e2b04b;
  --gold-soft: #c9973f;
  --teal:      #56b8ab;
  --rose:      #e07a7a;
  --blue:      #7aa5e0;
  --serif:     "Iowan Old Style","Palatino Linotype",Palatino,Georgia,"Times New Roman",serif;
  --mono:      "SF Mono","Cascadia Code",Consolas,"Liberation Mono",monospace;
}

*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;
  background:var(--bg);
  color:var(--ink);
  font-family:var(--serif);
  font-size:18px;
  line-height:1.75;
  -webkit-font-smoothing:antialiased;
}
::selection{background:rgba(226,176,75,.28)}

/* ---------------------------------------------------------------- top nav */
.topnav{
  position:sticky;top:0;z-index:50;
  display:flex;align-items:center;gap:1.25rem;
  padding:.65rem 1.5rem;
  background:rgba(14,17,23,.92);
  border-bottom:1px solid var(--line);
  backdrop-filter:blur(8px);
}
.topnav .brand{
  font-weight:700;letter-spacing:.04em;text-decoration:none;color:var(--gold);
  font-size:1.02rem;white-space:nowrap;
}
.topnav .brand .num{color:var(--ink);font-style:italic}
.topnav .links{display:flex;gap:1.1rem;flex-wrap:wrap}
.topnav a.crumb{color:var(--ink-dim);text-decoration:none;font-size:.92rem}
.topnav a.crumb:hover{color:var(--gold)}
.topnav .spacer{flex:1}
.topnav .prevnext{display:flex;gap:.6rem;margin-left:auto}
.topnav .prevnext a{
  color:var(--ink-dim);text-decoration:none;font-size:.88rem;
  border:1px solid var(--line);border-radius:999px;padding:.18rem .8rem;
}
.topnav .prevnext a:hover{border-color:var(--gold);color:var(--gold)}
.topnav .prevnext a.mute{opacity:.35;pointer-events:none}

/* ------------------------------------------------------------------ hero */
.hero{
  position:relative;
  padding:5.5rem 1.5rem 4rem;
  text-align:center;
  overflow:hidden;
}
.hero svg.backdrop{position:absolute;inset:0;width:100%;height:100%;opacity:.5}
.hero .inner{position:relative;max-width:880px;margin:0 auto}
.kicker{
  font-family:var(--mono);font-size:.78rem;letter-spacing:.32em;
  text-transform:uppercase;color:var(--teal);margin-bottom:1.4rem;
}
.hero h1{
  font-size:clamp(2.6rem,6vw,4.4rem);
  line-height:1.05;margin:0 0 1.2rem;font-weight:700;letter-spacing:-.01em;
}
.hero h1 .prob{color:var(--gold);font-style:italic}
.hero .tagline{
  font-size:1.3rem;color:var(--ink-dim);font-style:italic;
  max-width:36em;margin:0 auto 2.2rem;
}
.hero .lede{max-width:44em;margin:0 auto;text-align:left;color:var(--ink);font-size:1.05rem}
.hero .lede p{margin:0 0 1em}

/* ------------------------------------------------------------ typography */
main{display:block}
.wrap{max-width:920px;margin:0 auto;padding:0 1.5rem}
.wide{max-width:1080px;margin:0 auto;padding:0 1.5rem}
h1,h2,h3,h4{line-height:1.2;font-weight:700;letter-spacing:-.01em}
h2{font-size:2rem;margin:3.2rem 0 1rem;color:var(--ink)}
h2 .secno{color:var(--gold);margin-right:.5rem;font-style:italic}
h3{font-size:1.4rem;margin:2.4rem 0 .6rem;color:var(--ink)}
a{color:var(--gold)}
p{margin:0 0 1.15em}
em{color:var(--ink)}
.small{font-size:.9rem;color:var(--ink-dim)}
hr.sep{border:0;border-top:1px solid var(--line);margin:3.5rem 0}

/* chapter header */
.chapter-head{margin:3rem 0 0}
.chapter-head .chip{
  display:inline-block;font-family:var(--mono);font-size:.75rem;letter-spacing:.22em;
  text-transform:uppercase;color:var(--gold);
  border:1px solid var(--gold-soft);border-radius:999px;padding:.2rem .9rem;margin-bottom:1rem;
}
.chapter-head h1{font-size:clamp(2rem,4.5vw,3rem);margin:0 0 .6rem}
.chapter-head .standfirst{
  font-size:1.2rem;font-style:italic;color:var(--ink-dim);margin:0 0 2rem;max-width:40em;
}

/* table of contents */
.toc{
  background:var(--panel);border:1px solid var(--line);border-radius:14px;
  padding:1.4rem 1.8rem;margin:2.5rem 0;
}
.toc h4{margin:0 0 .8rem;font-family:var(--mono);font-size:.78rem;letter-spacing:.25em;
  text-transform:uppercase;color:var(--teal)}
.toc ol{margin:0;padding-left:1.4rem}
.toc li{margin:.35rem 0}
.toc a{color:var(--ink);text-decoration:none}
.toc a:hover{color:var(--gold)}

/* --------------------------------------------------------------- math css */
/* Math is typeset with Unicode + spans. No external renderers allowed. */
m, .m{font-style:italic;font-family:var(--serif)}
.mu, .rm{font-style:normal;font-family:var(--serif)}
.eq{
  display:block;margin:1.7rem auto;padding:1rem 1.4rem;
  text-align:center;font-size:1.18rem;line-height:1.9;
  background:var(--bg-soft);border:1px solid var(--line);border-radius:12px;
  overflow-x:auto;max-width:100%;
}
.eq .tag{float:right;color:var(--ink-faint);font-family:var(--mono);font-size:.75rem;padding-left:1rem}
.inl{font-family:var(--serif);font-style:italic;white-space:nowrap}
.inl .rm{font-style:normal}
.frac{display:inline-block;vertical-align:middle;text-align:center;line-height:1.2;margin:0 .15em}
.frac .nu{display:block;padding:.06em .3em;border-bottom:1.5px solid currentColor}
.frac .de{display:block;padding:.06em .3em}
.big{font-size:1.25em;vertical-align:middle;line-height:1}
ol.steps{counter-reset:step;list-style:none;padding-left:0}
ol.steps>li{counter-increment:step;position:relative;padding-left:2.6rem;margin:0 0 .9rem}
ol.steps>li::before{
  content:counter(step);position:absolute;left:0;top:.25rem;
  width:1.7rem;height:1.7rem;border-radius:50%;
  background:var(--panel-2);border:1px solid var(--line);
  color:var(--gold);font-family:var(--mono);font-size:.85rem;
  display:flex;align-items:center;justify-content:center;
}

/* ---------------------------------------------------------------- blocks */
.block{
  border:1px solid var(--line);border-left:4px solid var(--gold);
  border-radius:10px;background:var(--panel);
  padding:1.1rem 1.5rem;margin:1.8rem 0;
}
.block .bt{
  font-family:var(--mono);font-size:.74rem;letter-spacing:.22em;text-transform:uppercase;
  display:block;margin-bottom:.5rem;
}
.block .bt .name{font-style:italic;font-family:var(--serif);text-transform:none;letter-spacing:.02em;font-size:.92rem}
.bt::before{content:"";}
.def{border-left-color:var(--blue)}   .def .bt{color:var(--blue)}
.thm{border-left-color:var(--gold)}  .thm .bt{color:var(--gold)}
.pf{border-left-color:var(--ink-faint);background:transparent}
.pf .bt{color:var(--ink-faint)}
.pf::after{content:"∎";float:right;color:var(--ink-faint)}
.ex-pos{border-left-color:var(--teal)} .ex-pos .bt{color:var(--teal)}
.ex-neg{border-left-color:var(--rose)} .ex-neg .bt{color:var(--rose)}
.warn{border-left-color:var(--rose);background:rgba(224,122,122,.06)}
.warn .bt{color:var(--rose)}
.note{border-left-color:var(--gold-soft)} .note .bt{color:var(--gold-soft)}

/* two-column figure grid */
.figs{display:grid;grid-template-columns:repeat(auto-fit,minmax(280px,1fr));gap:1.2rem;margin:2rem 0}
figure{margin:0;background:var(--bg-soft);border:1px solid var(--line);border-radius:12px;padding:1rem}
figure svg{width:100%;height:auto;display:block}
figcaption{margin-top:.7rem;font-size:.88rem;color:var(--ink-dim);font-style:italic;text-align:center}

/* quote / pull */
.pull{
  border-left:3px solid var(--gold);padding:.4rem 0 .4rem 1.4rem;margin:2rem 0;
  font-size:1.25rem;font-style:italic;color:var(--ink);max-width:34em;
}

/* -------------------------------------------------------------- syllabus */
.syllabus{margin:2.5rem 0}
.chrow{
  display:grid;grid-template-columns:110px 1fr;gap:1.5rem;
  border-top:1px solid var(--line);padding:1.6rem 0;
}
.chrow .chno{
  font-style:italic;font-size:2rem;color:var(--gold);text-align:right;line-height:1;
}
.chrow .chno .small{display:block;font-style:normal;font-family:var(--mono);font-size:.68rem;
  letter-spacing:.2em;text-transform:uppercase;color:var(--ink-faint);margin-top:.5rem}
.chrow h3{margin:0 0 .4rem;font-size:1.3rem}
.chrow .desc{color:var(--ink-dim);font-size:.95rem;margin:0 0 .7rem}
ul.subs{list-style:none;padding:0;margin:0}
ul.subs li{margin:.3rem 0;font-size:.97rem}
ul.subs .code{font-family:var(--mono);font-size:.78rem;color:var(--teal);margin-right:.7rem}
ul.subs a{color:var(--ink);text-decoration:none;border-bottom:1px dotted var(--gold-soft)}
ul.subs a:hover{color:var(--gold)}
.chip-status{
  display:inline-block;font-family:var(--mono);font-size:.66rem;letter-spacing:.16em;
  text-transform:uppercase;border-radius:999px;padding:.1rem .6rem;margin-left:.5rem;
  border:1px solid var(--line);color:var(--ink-faint);vertical-align:middle;
}
.chip-status.live{color:var(--teal);border-color:var(--teal)}

/* cards */
.cards{display:grid;grid-template-columns:repeat(auto-fit,minmax(240px,1fr));gap:1.2rem;margin:2.2rem 0}
.card{
  background:var(--panel);border:1px solid var(--line);border-radius:14px;
  padding:1.4rem 1.5rem;text-decoration:none;color:var(--ink);display:block;
  transition:transform .15s ease,border-color .15s ease;
}
.card:hover{transform:translateY(-3px);border-color:var(--gold-soft)}
.card .cno{font-style:italic;font-size:1.6rem;color:var(--gold)}
.card h4{margin:.35rem 0 .45rem;font-size:1.1rem}
.card p{margin:0;font-size:.9rem;color:var(--ink-dim)}

/* footer */
footer.site{
  margin-top:5rem;border-top:1px solid var(--line);
  padding:2.5rem 1.5rem;text-align:center;color:var(--ink-faint);font-size:.85rem;
}
footer.site a{color:var(--ink-dim)}

@media (max-width:640px){
  body{font-size:16.5px}
  .chrow{grid-template-columns:1fr;gap:.4rem}
  .chrow .chno{text-align:left;font-size:1.6rem}
}