:root {
  --paper: #F7F4EC; --card: #FFFDF8; --ink: #1D2B3A; --muted: #5B6673; --line: #E2DCCD;
  --accent: #2E86AB; --accent-ink: #FFFFFF; --warm: #E8A33D; --good: #3A7D44;
  --radius: 14px; --measure: 46rem;
  color-scheme: light dark;
}
@media (prefers-color-scheme: dark) {
  :root { --paper: #12181F; --card: #1A222C; --ink: #E7E3D9; --muted: #A3ACB6; --line: #2C3742;
          --accent: #5BB0D6; --accent-ink: #0E1A22; }
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--paper); color: var(--ink);
       font: 17px/1.65 system-ui, -apple-system, "Segoe UI", Roboto, "Noto Sans", "Noto Sans Devanagari",
             "Noto Sans SC", sans-serif; }
main { max-width: var(--measure); margin: 0 auto; padding: 0 16px 48px; }
a { color: var(--accent); }
h1 { font-size: clamp(1.9rem, 5vw, 2.6rem); line-height: 1.15; margin: .4em 0 .3em; }
h2 { font-size: 1.5rem; margin: 2.2em 0 .6em; padding-top: .4em; border-top: 2px solid var(--line); }
h3 { font-size: 1.15rem; margin: 1.6em 0 .4em; }
p, li, dd { overflow-wrap: anywhere; }
.lead { font-size: 1.12rem; color: var(--muted); }
img { max-width: 100%; height: auto; }

.top { display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap;
       max-width: var(--measure); margin: 0 auto; padding: 12px 16px; }
.brand { display: flex; align-items: center; gap: 8px; font-weight: 700; color: var(--ink); text-decoration: none; }
.brand img { border-radius: 7px; }
.langs { position: relative; }
.langs summary { cursor: pointer; color: var(--muted); font-size: .95rem; list-style: none; }
.langs summary::-webkit-details-marker { display: none; }
.langs summary::after { content: " ▾"; }
.langs ul { position: absolute; right: 0; z-index: 2; margin: 6px 0 0; padding: 6px; list-style: none;
            background: var(--card); border: 1px solid var(--line); border-radius: 10px; min-width: 10rem;
            box-shadow: 0 8px 24px rgba(0,0,0,.12); }
.langs a { display: block; padding: 6px 10px; border-radius: 6px; text-decoration: none; color: var(--ink); }
.langs a:hover, .langs a[aria-current] { background: var(--paper); }

.crumbs { font-size: .9rem; color: var(--muted); margin-top: 8px; }
.crumbs a { color: var(--muted); }
.hero, .fig { margin: 1.4em 0; }
.hero img, .fig img { display: block; width: 100%; border-radius: var(--radius); background: #F7F4EC;
                      border: 1px solid var(--line); }
figcaption { font-size: .93rem; color: var(--muted); margin-top: .5em; }

.cta { background: var(--card); border: 1px solid var(--line); border-left: 5px solid var(--warm);
       border-radius: var(--radius); padding: 16px 18px; margin: 1.6em 0; }
.cta h2 { border: 0; margin: 0 0 .3em; padding: 0; font-size: 1.2rem; }
.cta p { margin: 0 0 12px; }
.play { display: inline-block; background: var(--accent); color: var(--accent-ink); font-weight: 700;
        text-decoration: none; padding: 10px 18px; border-radius: 999px; }
.play:hover { filter: brightness(1.08); }

.pre { color: var(--muted); }
.levels { position: sticky; top: 0; z-index: 1; display: flex; gap: 6px; overflow-x: auto; padding: 10px 0;
          background: var(--paper); border-bottom: 1px solid var(--line); scrollbar-width: none; }
.levels a { flex: none; padding: 6px 14px; border: 1px solid var(--line); border-radius: 999px;
            background: var(--card); text-decoration: none; color: var(--ink); font-size: .95rem; }
section { scroll-margin-top: 60px; }

.keys { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 4px 16px 10px; }
.keys h3 { margin-top: .8em; }
.keys ul { list-style: none; padding: 0; margin: 0; }
.keys li { display: flex; gap: 12px; align-items: flex-start; padding: 8px 0; border-top: 1px solid var(--line); }
.keys li:first-child { border-top: 0; }
.keys img { flex: none; width: 64px; height: auto; border-radius: 10px; background: #F7F4EC; }

.formula { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
           padding: 2px 16px 8px; margin: 1em 0; }
.formula h3 { margin-top: .8em; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; }
.katex { font-size: 1.05em; }
table { border-collapse: collapse; width: 100%; font-size: .93rem; margin: .6em 0; display: block; overflow-x: auto; }
caption { text-align: left; font-weight: 700; padding: 4px 0; }
td { border-top: 1px solid var(--line); padding: 6px 8px; vertical-align: top; }
td:first-child { white-space: nowrap; }
code.tex { font-size: .9em; }

dl dt { font-weight: 700; margin-top: .8em; }
dl dd { margin: 0 0 0 1em; }
.legend summary { cursor: pointer; font-size: .93rem; color: var(--muted); }

.quiz { padding-left: 1.4em; }
.quiz .q { margin-bottom: 1.4em; }
.quiz ol { margin: .3em 0; }
.quiz details summary { cursor: pointer; color: var(--accent); font-weight: 600; }
.tag { display: inline-block; font-size: .75rem; padding: 1px 8px; border-radius: 999px; background: var(--line);
       color: var(--muted); vertical-align: middle; }

.pager { display: flex; justify-content: space-between; gap: 12px; margin-top: 2em; }
.pager a { flex: 1; padding: 12px 14px; border: 1px solid var(--line); border-radius: var(--radius);
           background: var(--card); text-decoration: none; color: var(--ink); }
.pager a:last-child { text-align: right; }
.pager small { display: block; color: var(--muted); }

.topics { list-style: none; padding: 0; display: grid; grid-template-columns: repeat(auto-fill, minmax(16rem, 1fr)); gap: 10px; }
.topics a { display: flex; gap: 12px; align-items: flex-start; height: 100%; padding: 12px; background: var(--card);
            border: 1px solid var(--line); border-radius: var(--radius); text-decoration: none; color: var(--ink); }
.topics img { flex: none; border-radius: 10px; background: #F7F4EC; }
.topics b { display: block; }
.topics small { display: -webkit-box; -webkit-line-clamp: 3; -webkit-box-orient: vertical; overflow: hidden;
                color: var(--muted); font-size: .85rem; line-height: 1.45; }

.root { text-align: center; padding-top: 12vh; }
.pick { list-style: none; padding: 0; display: grid; gap: 8px; max-width: 22rem; margin: 1.5em auto; }
.pick a { display: block; padding: 12px; border: 1px solid var(--line); border-radius: var(--radius);
          background: var(--card); text-decoration: none; color: var(--ink); }

footer { max-width: var(--measure); margin: 0 auto; padding: 24px 16px 40px; color: var(--muted); font-size: .9rem;
         border-top: 1px solid var(--line); }