:root { --c-eps: #7A5C00; --c-ok: #0A7C74; }
@media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) { --c-eps: #E8C766; } }
:root[data-theme="dark"] { --c-eps: #E8C766; }

.lesson-hero { padding-bottom: 8px; }
.ctl2 label .kv-e, .ctl2 label .kv-a, .demo label .kv-e, .demo label .kv-a { text-transform: none; font-family: var(--f-body); font-size: 1.05em; }
/* Mathematical symbols keep their case inside uppercase labels, headings and table headers. */
.m, .m *, .katex, .katex *, .sy, .sy *, .lc, .lc *, .kv-e, .kv-a, .kv-e *, .kv-a * { text-transform: none; }
.lsec { scroll-margin-top: 90px; }
.chips { display: flex; flex-wrap: wrap; gap: 8px 10px; margin-top: 22px; }
.chip { font-family: var(--f-mono); font-size: 0.74rem; letter-spacing: 0.04em; border: 1px solid var(--line); padding: 5px 10px; border-radius: 2px; color: var(--muted); }
.lesson-grid { display: grid; grid-template-columns: 210px minmax(0, 1fr); gap: 0 clamp(28px, 4vw, 64px); margin-top: 36px; }
.toc { position: sticky; top: 20px; align-self: start; font-family: var(--f-mono); font-size: 0.76rem; letter-spacing: 0.03em; display: grid; gap: 4px; max-height: calc(100vh - 40px); overflow: auto; }
.toc b { font-weight: 500; text-transform: uppercase; letter-spacing: 0.1em; color: var(--muted); margin-bottom: 6px; }
.toc a { text-decoration: none; color: var(--muted); padding: 5px 0 5px 12px; border-left: 2px solid var(--line); line-height: 1.35; }
.toc a:hover, .toc a.on { color: var(--ink); border-left-color: var(--accent); }
@media (max-width: 1000px) { .lesson-grid { grid-template-columns: minmax(0, 1fr); } .toc { display: none; } }

.lsec { padding-top: 56px; margin-top: 56px; border-top: 1px solid var(--line); min-width: 0; }
.lsec:first-child { border-top: 0; margin-top: 0; padding-top: 0; }
.lsec h2 { font-size: clamp(1.8rem, 4vw, 2.6rem); margin: 0 0 18px; }
.lsec h3 { font-size: 1.35rem; margin: 32px 0 10px; }
.lsec > p, .lsec > ul, .lsec > ol, .lsec .prose-in { max-width: 66ch; }
.lsec > p { margin: 0 0 1em; }
.snum { font-family: var(--f-mono); font-size: 0.78rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); display: block; margin-bottom: 8px; }
.callout-b { border-left: 3px solid var(--accent); background: var(--surface); padding: 14px 18px; margin: 20px 0; max-width: 70ch; }
.callout-b p { margin: 0 0 0.6em; } .callout-b p:last-child { margin: 0; }
.callout-b b.t { font-family: var(--f-mono); font-weight: 500; font-size: 0.74rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); display: block; margin-bottom: 6px; }

.eqblock { margin: 22px 0; max-width: 100%; }
.eqrow { display: flex; align-items: center; justify-content: space-between; gap: 16px; border: 1px solid var(--line); background: var(--surface); padding: 6px 18px; overflow-x: auto; }
.eq { flex: 1 1 auto; min-width: 0; text-align: center; font-size: 1.28em; }
.eqn { font-family: var(--f-mono); font-size: 0.82rem; color: var(--muted); flex: none; }
.gloss { border: 1px solid var(--line); border-top: 0; padding: 12px 18px; font-size: 0.97rem; min-height: 3.2em; background: var(--bg); }
.gloss .hint-s { color: var(--muted); }
.gloss b { font-family: var(--f-display); font-weight: 800; font-stretch: 90%; }
[class*="s-"] { cursor: pointer; border-radius: 2px; transition: background .12s; }
.c-gd { color: var(--t-gd); } .c-mom { color: var(--t-mom); } .c-acc { color: var(--accent); } .c-eps { color: var(--c-eps); } .c-ink { color: var(--ink); }
.symon { background: color-mix(in srgb, var(--accent) 18%, transparent); outline: 1px solid color-mix(in srgb, var(--accent) 45%, transparent); }
.kv-v { color: var(--t-mom); } .kv-a { color: var(--accent); } .kv-e { color: var(--c-eps); } .kv-g { color: var(--t-gd); } .kv-b, .kv-i, .kv-n, .kv-s { color: var(--ink); }
.key-row { display: flex; flex-wrap: wrap; gap: 10px 24px; margin: 16px 0; font-size: 0.97rem; }
.key-row span { display: inline-flex; gap: 8px; align-items: baseline; }
.key-row .sw { display: inline-block; width: 12px; height: 12px; border-radius: 2px; transform: translateY(1px); }

.demo { border: 1px solid var(--line); background: var(--surface); padding: 20px; margin: 26px 0; display: grid; gap: 16px; border-radius: 3px; min-width: 0; }
.demo > h4 { margin: 0; font-family: var(--f-mono); font-weight: 500; font-size: 0.78rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); }
.demo canvas { display: block; width: 100%; background: var(--bg); outline: 1px solid var(--line); outline-offset: -1px; }
.dgrid { display: grid; grid-template-columns: minmax(0, 1.5fr) minmax(0, 1fr); gap: 20px 28px; align-items: start; }
@media (max-width: 900px) { .dgrid { grid-template-columns: minmax(0, 1fr); } }
.ctls2 { display: grid; gap: 14px; }
.ctl2 { display: grid; gap: 4px; }
.ctl2 .top2 { display: flex; justify-content: space-between; gap: 10px; align-items: baseline; }
.ctl2 label { font-family: var(--f-mono); font-size: 0.74rem; letter-spacing: 0.06em; text-transform: uppercase; }
.ctl2 .val2 { font-family: var(--f-mono); font-size: 0.84rem; color: var(--muted); }
.demo input[type="range"] { width: 100%; accent-color: var(--accent); }
.demo input[type="number"] { font: inherit; font-family: var(--f-mono); width: 100%; padding: 8px 10px; background: var(--bg); color: var(--ink); border: 1px solid var(--line); border-radius: 2px; }
.btnrow { display: flex; flex-wrap: wrap; gap: 8px; }
.demo .readout { margin: 0; padding: 12px 14px; background: var(--bg); border: 1px solid var(--line); font-size: 0.97rem; }
.demo .readout b { font-family: var(--f-mono); font-weight: 500; }
.chk { display: flex; gap: 8px; align-items: center; font-size: 0.93rem; }
.chk input { accent-color: var(--accent); }
.tabs { display: flex; flex-wrap: wrap; border: 1px solid var(--line); width: fit-content; max-width: 100%; border-radius: 2px; overflow: hidden; }
.tabs button { font: inherit; font-family: var(--f-mono); font-size: 0.76rem; padding: 9px 14px; border: 0; background: transparent; color: var(--ink); cursor: pointer; }
.tabs button + button { border-left: 1px solid var(--line); }
.tabs button.on { background: var(--accent); color: var(--accent-ink); }
.tabs button.on .katex, .tabs button.on .katex * { color: var(--accent-ink) !important; }
.tabs button { flex: 1 1 auto; white-space: nowrap; }
.tabs { width: 100%; }
.lg { display: flex; flex-wrap: wrap; gap: 6px 20px; font-family: var(--f-mono); font-size: 0.78rem; }
.lg i { display: inline-block; width: 18px; height: 3px; margin-right: 8px; vertical-align: middle; }

.algo { border-top: 2px solid var(--ink); border-bottom: 2px solid var(--ink); background: var(--bg); padding: 8px 0; font-size: 0.97rem; }
.algo .ln { display: grid; grid-template-columns: 2.2rem minmax(0, 1fr); padding: 4px 12px; border-left: 3px solid transparent; }
.algo .ln .no { font-family: var(--f-mono); font-size: 0.74rem; color: var(--muted); padding-top: 4px; }
.algo .ln.on { background: color-mix(in srgb, var(--accent) 14%, transparent); border-left-color: var(--accent); }
.algo .ind1 { padding-left: 1.6rem; } .algo .ind2 { padding-left: 1.6rem; }
.tbl { width: 100%; border-collapse: collapse; font-family: var(--f-mono); font-size: 0.8rem; }
.tbl th, .tbl td { border-bottom: 1px solid var(--line); padding: 7px 8px; text-align: right; white-space: nowrap; }
.tbl th { color: var(--muted); font-weight: 500; text-transform: uppercase; letter-spacing: 0.06em; font-size: 0.7rem; }
.tbl th:first-child, .tbl td:first-child { text-align: left; }
.tblwrap { overflow-x: auto; }
.calc { font-family: var(--f-mono); font-size: 0.85rem; display: grid; gap: 6px; margin: 0; padding: 12px 14px; background: var(--bg); border: 1px solid var(--line); overflow-x: auto; }
.calc div { white-space: nowrap; }
.bars2 { display: grid; gap: 4px; }
.quiz { display: grid; gap: 22px; margin-top: 18px; }
.q { border: 1px solid var(--line); padding: 16px 18px; background: var(--surface); }
.q p.qq { margin: 0 0 12px; font-weight: 600; max-width: 70ch; }
.q .opts2 { display: grid; gap: 8px; }
.q button.o { text-align: left; font: inherit; padding: 10px 14px; border: 1px solid var(--line); background: var(--bg); color: var(--ink); cursor: pointer; border-radius: 2px; }
.q button.o:hover { border-color: var(--ink); }
.q button.o.right { border-color: var(--c-ok); box-shadow: inset 0 0 0 1px var(--c-ok); }
.q button.o.wrong { border-color: var(--danger); box-shadow: inset 0 0 0 1px var(--danger); }
.q .why { margin: 12px 0 0; color: var(--muted); font-size: 0.95rem; }
.summary { display: grid; gap: 10px; padding-left: 1.2em; max-width: 66ch; }
.srcs { font-size: 0.92rem; color: var(--muted); max-width: 70ch; }

/* ---- learn bar, checkpoints, completion, toast ---- */
#learn-bar { border: 1px solid var(--line); background: var(--surface); padding: 12px 16px; margin: 26px 0 0; border-radius: 3px; display: grid; gap: 10px; }
#learn-bar .bar-top { display: flex; flex-wrap: wrap; gap: 6px 18px; align-items: baseline; justify-content: space-between; }
#learn-bar .mode { font-family: var(--f-mono); font-size: 0.74rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); }
#learn-bar .where { font-size: 0.93rem; color: var(--muted); flex: 1 1 14rem; }
.hud { display: inline-flex; flex-wrap: wrap; align-items: center; gap: 6px 12px; font-family: var(--f-mono); font-size: 0.76rem; }
.hud .lvl b { font-weight: 500; color: var(--ink); }
.xpbar { display: inline-block; width: 90px; height: 6px; background: var(--line); border-radius: 3px; overflow: hidden; }
.xpbar i { display: block; height: 100%; background: var(--accent); }
.hud .stk { color: var(--t-gd); }
.segs { display: flex; gap: 2px; height: 14px; }
.segs .seg { flex: 1 1 0; min-width: 2px; background: var(--line); border-radius: 1px; }
.segs .seg.part { background: color-mix(in srgb, var(--accent) 40%, var(--line)); }
.segs .seg.good { background: var(--accent); }
.segs .seg.review { background: var(--c-eps); }
.segs .seg.cur { outline: 2px solid var(--ink); outline-offset: 1px; }
.segs.one { height: 8px; background: var(--line); border-radius: 4px; overflow: hidden; display: block; }
.segs.one .fill { display: block; height: 100%; background: var(--accent); }
.bar-sub { display: flex; flex-wrap: wrap; gap: 6px 20px; font-size: 0.88rem; color: var(--muted); align-items: baseline; }
.bar-sub a, .linkish { color: var(--accent); }
.linkish { font: inherit; background: none; border: 0; padding: 0; cursor: pointer; text-decoration: underline; text-underline-offset: 3px; }
.check { border-left: 3px solid var(--c-eps); background: var(--surface); padding: 14px 18px; margin: 34px 0 0; max-width: 70ch; display: grid; gap: 14px; }
.check > b.t { font-family: var(--f-mono); font-weight: 500; font-size: 0.74rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--c-eps); }
.check .cq { display: grid; gap: 14px; }
.check .q { border: 0; padding: 0; background: transparent; }
.q.answered .qq { color: var(--muted); }
.q button.o:disabled { cursor: default; opacity: 0.92; }
#module-done { border: 1px solid var(--accent); background: var(--surface); padding: 20px 22px; margin: 28px 0; max-width: 70ch; }
#module-done .t { font-family: var(--f-mono); font-weight: 500; font-size: 0.74rem; letter-spacing: 0.1em; text-transform: uppercase; color: var(--accent); display: block; margin-bottom: 8px; }
#module-done .big { font-size: 1.15rem; margin: 0 0 8px; }
#xp-toast { position: fixed; right: 18px; bottom: 18px; background: var(--ink); color: var(--bg); font-family: var(--f-mono); font-size: 0.82rem; padding: 10px 14px; border-radius: 3px; opacity: 0; transform: translateY(8px); transition: opacity .2s, transform .2s; pointer-events: none; z-index: 50; max-width: 80vw; }
#xp-toast.show { opacity: 1; transform: none; }
.lsec { scroll-margin-top: 90px; }
.eq-note { font-size: 0.93rem; color: var(--muted); max-width: 70ch; }
.demo .note { margin: 0; color: var(--muted); font-size: 0.9rem; }
.lsec .figs { display: grid; gap: 16px; }
.grid2 { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 16px; }
@media (max-width: 700px) { .grid2 { grid-template-columns: minmax(0, 1fr); } }
.mono-in { font-family: var(--f-mono); }
.tag { font-family: var(--f-mono); font-size: 0.72rem; letter-spacing: 0.06em; text-transform: uppercase; color: var(--muted); }

/* ---- hub ---- */
.hub-choices { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 18px; margin-top: 30px; }
@media (max-width: 760px) { .hub-choices { grid-template-columns: minmax(0, 1fr); } }
.choice { border: 1px solid var(--line); background: var(--surface); padding: 24px; border-radius: 3px; display: grid; gap: 12px; align-content: start; }
.choice h2 { margin: 0; font-size: 1.5rem; }
.choice p { margin: 0; max-width: 52ch; }
.choice.main { border-color: var(--accent); }
.profile { margin-top: 22px; border: 1px solid var(--line); padding: 18px 22px; display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 14px 22px; border-radius: 3px; }
@media (max-width: 760px) { .profile { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
.profile .pst b { display: block; font-family: var(--f-display); font-weight: 800; font-stretch: 90%; font-size: 1.6rem; line-height: 1.1; }
.profile .pst span { font-family: var(--f-mono); font-size: 0.72rem; letter-spacing: 0.08em; text-transform: uppercase; color: var(--muted); }
.partbox { margin-top: 56px; }
.partbox > h2 { font-size: 1.4rem; margin: 0 0 4px; }
.partbox > .tag { display: block; margin-bottom: 6px; }
.chbox { border-top: 1px solid var(--line); padding-top: 26px; margin-top: 26px; }
.chbox h3 { margin: 0 0 4px; font-size: 1.25rem; }
.chbox .chmeta { color: var(--muted); font-size: 0.93rem; margin: 0 0 14px; }
.mgrid { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(100%, 270px), 1fr)); gap: 12px; }
.mcard { border: 1px solid var(--line); background: var(--surface); padding: 14px 16px; border-radius: 3px; display: grid; gap: 6px; align-content: start; text-decoration: none; color: var(--ink); position: relative; }
.mcard:hover { border-color: var(--ink); }
.mcard .sec { font-family: var(--f-mono); font-size: 0.74rem; letter-spacing: 0.08em; color: var(--accent); }
.mcard h4 { margin: 0; font-family: var(--f-display); font-weight: 700; font-stretch: 88%; font-size: 1.08rem; line-height: 1.2; }
.mcard p { margin: 0; font-size: 0.9rem; color: var(--muted); }
.mcard .meta { font-family: var(--f-mono); font-size: 0.7rem; color: var(--muted); letter-spacing: 0.04em; }
.mcard .subs { display: flex; flex-wrap: wrap; gap: 4px 6px; margin-top: 2px; }
.mcard .subs span { font-size: 0.74rem; border: 1px solid var(--line); padding: 1px 6px; border-radius: 2px; color: var(--muted); }
.mcard .mbar { height: 5px; background: var(--line); border-radius: 3px; overflow: hidden; margin-top: 4px; }
.mcard .mbar i { display: block; height: 100%; background: var(--accent); }
.mcard.done .mbar i { background: var(--accent); }
.mcard.review .mbar i { background: var(--c-eps); }
.mcard .badge { position: absolute; top: 10px; right: 12px; font-family: var(--f-mono); font-size: 0.7rem; letter-spacing: 0.06em; color: var(--accent); }
.mcard.review .badge { color: var(--c-eps); }
.soon { border: 1px dashed var(--line); padding: 12px 16px; border-radius: 3px; color: var(--muted); font-size: 0.93rem; }
.soon ul { margin: 6px 0 0; padding-left: 1.1em; columns: 2; column-gap: 28px; }
@media (max-width: 700px) { .soon ul { columns: 1; } }
.hub-foot { margin-top: 48px; display: flex; flex-wrap: wrap; gap: 10px 20px; align-items: center; }

/* ---- long equations on phones: scroll inside the row, number above ---- */
.eqrow { max-width: 100%; }
.eq { overflow-x: auto; overflow-y: hidden; max-width: 100%; }
@media (max-width: 640px) {
  .eqrow { flex-direction: column-reverse; align-items: stretch; gap: 2px; padding: 6px 12px; }
  .eqn { align-self: flex-end; }
  .eq { font-size: 1.1em; }
}
.demo > * { min-width: 0; }
.dgrid > * { min-width: 0; }
@media (max-width: 640px) { .calc div { white-space: normal; overflow-wrap: anywhere; } }

/* clickable symbols in running text: dotted underline cue, floating explanation */
.m [class*="s-"] { text-decoration: underline dotted; text-decoration-thickness: 1.5px; text-underline-offset: 3px; }
[class*="s-"]:hover { background: color-mix(in srgb, var(--accent) 12%, transparent); }
.sympop { position: absolute; z-index: 900; max-width: min(340px, calc(100vw - 16px)); background: var(--bg); border: 1px solid var(--accent); box-shadow: 0 10px 30px rgba(0,0,0,.22); padding: 12px 34px 12px 14px; font-size: .95rem; line-height: 1.45; border-radius: 3px; }
.sympop .x { position: absolute; top: 4px; right: 6px; border: 0; background: none; color: var(--muted); font-size: 1.2rem; cursor: pointer; line-height: 1; }
.sympop b { font-family: var(--f-display); font-weight: 800; font-stretch: 90%; }
/* challenges */
.challenge { display: flex; gap: 14px; align-items: flex-start; border: 1.5px dashed var(--accent); background: color-mix(in srgb, var(--accent) 6%, var(--bg)); padding: 14px 18px; margin: 22px 0; border-radius: 3px; max-width: 70ch; }
.challenge .ck { flex: none; width: 26px; height: 26px; border-radius: 50%; border: 2px solid var(--accent); display: grid; place-items: center; font-size: .9rem; color: transparent; margin-top: 2px; }
.challenge b.t { display: block; font-family: var(--f-mono); font-weight: 500; font-size: .74rem; letter-spacing: .1em; text-transform: uppercase; color: var(--accent); margin-bottom: 4px; }
.challenge p { margin: 0; }
.challenge.solved { border-style: solid; background: color-mix(in srgb, var(--accent) 14%, var(--bg)); }
.challenge.solved .ck { background: var(--accent); color: var(--bg); }
.challenge.solved .ck::after { content: "\2713"; }
.challenge .xpt { font-family: var(--f-mono); font-size: .72rem; color: var(--muted); margin-left: 6px; }
details.deeper { border: 1px solid var(--line); background: var(--surface); margin: 18px 0; max-width: 70ch; border-radius: 3px; }
details.deeper > summary { cursor: pointer; padding: 10px 16px; font-family: var(--f-mono); font-size: .76rem; letter-spacing: .1em; text-transform: uppercase; color: var(--accent); list-style: none; }
details.deeper > summary::before { content: "+ "; } details.deeper[open] > summary::before { content: "\2212 "; }
details.deeper > div { padding: 2px 18px 14px; }