:root{
  --bg:#f6f7f9;
  --card-bg:#ffffff;
  --ink:#22262e;
  --ink-soft:#5b6472;
  --line:#e2e5ea;
  --brand:#1f6f5c;
  --brand-soft:#e5f2ee;
  --accent:#c95f26;
  --accent-soft:#fbead9;
  --code-bg:#1e2530;
  --code-ink:#e7ecf3;
  --nav-bg:rgba(255,255,255,0.92);
  --thead-bg:#f0f2f5;
  --chip-bg:#eef0f3;
  --radius:14px;
  --maxw:920px;
}

@media (prefers-color-scheme: dark){
  :root:not([data-theme="light"]){
    --bg:#14171c;
    --card-bg:#1b1f26;
    --ink:#e6e9ee;
    --ink-soft:#9aa3b2;
    --line:#2a2f38;
    --brand:#4db69b;
    --brand-soft:rgba(77,182,155,0.14);
    --accent:#e2895a;
    --accent-soft:rgba(226,137,90,0.14);
    --code-bg:#10141a;
    --code-ink:#e7ecf3;
    --nav-bg:rgba(20,23,28,0.86);
    --thead-bg:#20252d;
    --chip-bg:#242a33;
  }
}

*{box-sizing:border-box;}
html{scroll-behavior:smooth;}
body{
  margin:0;
  background:var(--bg);
  color:var(--ink);
  font-family:"Segoe UI",Roboto,Helvetica,Arial,sans-serif;
  line-height:1.65;
}

img{max-width:100%;}

a{color:var(--brand);}
a:hover{color:var(--accent);}

.wrap{max-width:var(--maxw);margin:0 auto;padding:0 24px;}

/* ---------- top nav ---------- */
header.site-nav{
  position:sticky;top:0;z-index:50;
  background:var(--nav-bg);
  backdrop-filter:blur(6px);
  border-bottom:1px solid var(--line);
}
.nav-inner{
  max-width:1180px;margin:0 auto;padding:0 24px;
  display:flex;align-items:center;justify-content:space-between;
  height:60px;
}
.brand{
  font-weight:700;font-size:1.05rem;color:var(--ink);text-decoration:none;
  display:flex;align-items:center;gap:8px;
}
.brand .glyph{
  display:inline-flex;align-items:center;justify-content:center;
  width:30px;height:30px;border-radius:8px;
  background:var(--brand);color:#fff;font-size:1rem;font-weight:700;
}
nav.links{display:flex;gap:2px;flex-wrap:wrap;}
nav.links a{
  color:var(--ink-soft);text-decoration:none;font-size:0.86rem;
  padding:8px 10px;border-radius:8px;white-space:nowrap;
}
nav.links a:hover{background:var(--brand-soft);color:var(--brand);}
#nav-toggle{display:none;}
.burger{display:none;cursor:pointer;border:1px solid var(--line);border-radius:8px;padding:8px 10px;background:var(--card-bg);}
.burger span{display:block;width:20px;height:2px;background:var(--ink);margin:4px 0;}

@media (max-width:820px){
  nav.links{
    position:absolute;top:60px;left:0;right:0;
    background:var(--card-bg);border-bottom:1px solid var(--line);
    flex-direction:column;padding:8px;
    display:none;
  }
  nav.links a{padding:12px 14px;}
  #nav-toggle:checked ~ nav.links{display:flex;}
  .burger{display:block;}
}

/* ---------- hero ---------- */
.hero{
  padding:64px 0 48px;
  background:
    radial-gradient(600px 300px at 85% -10%, var(--brand-soft), transparent 60%),
    radial-gradient(500px 260px at 5% 0%, var(--accent-soft), transparent 60%);
}
.hero h1{font-size:clamp(1.9rem,4vw,2.7rem);margin:0 0 12px;letter-spacing:-0.01em;}
.hero p.lead{font-size:1.1rem;color:var(--ink-soft);max-width:62ch;margin:0 0 22px;}
.hero .formula-badge{
  display:inline-flex;align-items:center;gap:10px;
  background:var(--card-bg);border:1px solid var(--line);
  border-radius:999px;padding:10px 18px;font-size:1.05rem;
  box-shadow:0 1px 2px rgba(20,20,30,0.04);
}

/* ---------- sections ---------- */
section{padding:48px 0;border-bottom:1px solid var(--line);}
section:last-of-type{border-bottom:none;}
section h2{
  font-size:1.5rem;margin:0 0 6px;
  display:flex;align-items:center;gap:10px;
}
section h2 .tag{
  font-size:0.7rem;font-weight:700;letter-spacing:0.06em;text-transform:uppercase;
  color:var(--brand);background:var(--brand-soft);padding:4px 9px;border-radius:6px;
}
section > p.kicker{color:var(--ink-soft);margin:0 0 26px;max-width:70ch;}
h3{font-size:1.12rem;margin:28px 0 10px;}
h4{font-size:1rem;margin:20px 0 8px;color:var(--ink);}

.card{
  background:var(--card-bg);border:1px solid var(--line);border-radius:var(--radius);
  padding:22px 24px;margin:16px 0;
}
.grid-2{display:grid;grid-template-columns:1fr 1fr;gap:18px;}
@media (max-width:720px){.grid-2{grid-template-columns:1fr;}}

.timeline{list-style:none;margin:0;padding:0;border-left:2px solid var(--line);}
.timeline li{position:relative;padding:2px 0 22px 24px;}
.timeline li::before{
  content:"";position:absolute;left:-7px;top:5px;width:12px;height:12px;
  border-radius:50%;background:var(--brand);border:2px solid #fff;box-shadow:0 0 0 2px var(--brand);
}
.timeline .when{font-weight:700;color:var(--brand);display:block;font-size:0.92rem;}

pre{
  background:var(--code-bg);color:var(--code-ink);
  border-radius:10px;padding:16px 18px;overflow-x:auto;
  font-size:0.86rem;line-height:1.55;
}
code{font-family:"SFMono-Regular",Consolas,"Liberation Mono",Menlo,monospace;}
p code, li code{
  background:var(--brand-soft);color:var(--brand);padding:1px 6px;border-radius:5px;
  font-size:0.88em;
}
.code-tabs{display:flex;gap:6px;margin:18px 0 -1px;flex-wrap:wrap;}
.code-tabs span{
  font-size:0.78rem;font-weight:700;text-transform:uppercase;letter-spacing:0.04em;
  color:var(--ink-soft);background:var(--chip-bg);padding:6px 12px;border-radius:8px 8px 0 0;
}

.formula{
  font-size:1.15rem;text-align:center;padding:18px;
  background:var(--brand-soft);border-radius:10px;margin:16px 0;
  overflow-x:auto;
}
.formula i{font-style:italic;}
sup, sub{font-size:0.68em;}

/* MathJax SVG output: let it scroll instead of overflowing on narrow screens */
mjx-container{overflow-x:auto;overflow-y:hidden;max-width:100%;}
.stat .num mjx-container{display:inline-block;}

table{width:100%;border-collapse:collapse;margin:14px 0;font-size:0.94rem;}
th,td{padding:10px 12px;border-bottom:1px solid var(--line);text-align:right;}
th:first-child,td:first-child{text-align:left;}
thead th{background:var(--thead-bg);font-weight:700;}
tr.avg td{font-weight:700;background:var(--brand-soft);}

.badge-list{display:flex;gap:8px;flex-wrap:wrap;margin:10px 0;}
.badge-list li{
  list-style:none;background:var(--chip-bg);color:var(--ink-soft);
  padding:6px 12px;border-radius:999px;font-size:0.82rem;
}

.stat-row{display:flex;gap:16px;flex-wrap:wrap;margin:18px 0;}
.stat{
  flex:1;min-width:150px;background:var(--card-bg);border:1px solid var(--line);
  border-radius:12px;padding:16px 18px;
}
.stat .num{font-size:1.6rem;font-weight:700;color:var(--brand);}
.stat .lbl{font-size:0.82rem;color:var(--ink-soft);}

.callout{
  border-left:4px solid var(--accent);background:var(--accent-soft);
  border-radius:0 10px 10px 0;padding:14px 18px;margin:16px 0;
}
.callout.good{border-left-color:var(--brand);background:var(--brand-soft);}

ol.steps{padding-left:22px;}
ol.steps li{margin-bottom:8px;}

.img-frame{
  background:var(--card-bg);border:1px solid var(--line);border-radius:var(--radius);
  padding:12px;margin:18px 0;
}
.img-frame figcaption{
  text-align:center;font-size:0.85rem;color:var(--ink-soft);margin-top:8px;
}

footer{padding:32px 0 60px;color:var(--ink-soft);font-size:0.86rem;text-align:center;}

.sources ol{padding-left:20px;}
.sources li{margin-bottom:10px;}

.excel-shot{
  border:1px solid var(--line);border-radius:10px;overflow:hidden;
}
.excel-shot .row{display:grid;grid-template-columns:60px 1fr 1fr;font-size:0.82rem;}
.excel-shot .row > div{padding:6px 10px;border-bottom:1px solid var(--line);border-right:1px solid var(--line);}
.excel-shot .row.head > div{background:var(--brand);color:#fff;font-weight:700;}
.excel-shot .row > div:last-child{border-right:none;font-family:monospace;}

/* ---------- scroll-reveal ---------- */
.reveal{opacity:0;transform:translateY(16px);transition:opacity .55s ease, transform .55s ease;}
.reveal.in-view{opacity:1;transform:none;}
@media (prefers-reduced-motion: reduce){
  .reveal{opacity:1;transform:none;transition:none;}
}

/* ---------- code copy button ---------- */
pre{position:relative;}
.copy-btn{
  position:absolute;top:10px;right:10px;
  background:rgba(255,255,255,0.08);color:var(--code-ink);
  border:1px solid rgba(255,255,255,0.18);border-radius:6px;
  font-size:0.72rem;padding:5px 10px;cursor:pointer;
  font-family:inherit;letter-spacing:0.02em;
}
.copy-btn:hover{background:rgba(255,255,255,0.16);}
.copy-btn.copied{background:var(--brand);border-color:var(--brand);color:#fff;}

/* ---------- live calculator ---------- */
.calc-card{padding-top:20px;}
.calc-controls{
  display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin-bottom:18px;
}
.calc-controls label{font-weight:700;font-size:0.92rem;}
.calc-controls input{
  font:inherit;font-size:1rem;padding:9px 12px;border:1px solid var(--line);
  border-radius:8px;background:var(--bg);color:var(--ink);width:160px;
}
.calc-controls input:focus{outline:2px solid var(--brand);outline-offset:1px;}
.calc-controls button{
  font:inherit;font-weight:700;font-size:0.92rem;padding:10px 18px;
  background:var(--brand);color:#fff;border:none;border-radius:8px;cursor:pointer;
}
.calc-controls button:hover{filter:brightness(1.08);}
.calc-hint{color:var(--ink-soft);font-size:0.85rem;margin:-8px 0 16px;}
.calc-table{max-height:340px;overflow-y:auto;display:block;}
.calc-table thead,.calc-table tbody{display:table;width:100%;table-layout:fixed;}
.calc-table tbody tr:last-child td{font-weight:700;color:var(--brand);}