:root {
  color-scheme: light;
  --bg: #f6f3ec;
  --paper: #fffdf8;
  --ink: #1d2230;
  --muted: #5e6475;
  --line: #e2dccf;
  --accent: #2f55d4;
  --accent-soft: #e6ecff;
  --gold: #b7791f;
  --good: #1f8a5b;
  --bad: #c23b4e;
  --warn-bg: #fff4e3;
  --warn-line: #f0c27a;
  --A: #2f6fd4; --B: #7d4fd0; --C: #138a6c; --D: #c8601d;
  --s-number: #4f46e5; --s-algebra: #0f8f86; --s-geometry: #d0661c; --s-statistics: #c03a6b;
  --radius: 14px;
  --font-ui: "Atkinson Hyperlegible", system-ui, sans-serif;
  --font-head: "Fraunces", Georgia, serif;
  --font-math: "STIX Two Text", "Cambria Math", "Times New Roman", serif;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;
    --bg: #14171f; --paper: #1b1f2a; --ink: #e8eaf1; --muted: #a2a8ba; --line: #2d3342;
    --accent: #8ea8ff; --accent-soft: #232c48; --gold: #f0c674; --good: #5fd39b; --bad: #ff7d8f;
    --warn-bg: #2c2418; --warn-line: #7a5a26;
    --A: #7fb0ff; --B: #b99bff; --C: #5fd3b4; --D: #ffa86a;
    --s-number: #9b95ff; --s-algebra: #4fd1c5; --s-geometry: #ffa25a; --s-statistics: #ff86b0;
  }
}
:root[data-theme="dark"] {
  color-scheme: dark;
  --bg: #14171f; --paper: #1b1f2a; --ink: #e8eaf1; --muted: #a2a8ba; --line: #2d3342;
  --accent: #8ea8ff; --accent-soft: #232c48; --gold: #f0c674; --good: #5fd39b; --bad: #ff7d8f;
  --warn-bg: #2c2418; --warn-line: #7a5a26;
  --A: #7fb0ff; --B: #b99bff; --C: #5fd3b4; --D: #ffa86a;
  --s-number: #9b95ff; --s-algebra: #4fd1c5; --s-geometry: #ffa25a; --s-statistics: #ff86b0;
}
* { box-sizing: border-box; }
html, body { margin: 0; }
body { background: var(--bg); color: var(--ink); font: 17px/1.6 var(--font-ui); min-height: 100vh; display: flex; flex-direction: column; }
a { color: var(--accent); }
button { font: inherit; color: inherit; }
h1, h2, h3 { font-family: var(--font-head); line-height: 1.2; margin: 0 0 .5em; overflow-wrap: break-word; }
h1 { font-size: clamp(1.7rem, 4.5vw, 2.4rem); }
h2 { font-size: 1.45rem; margin-top: 1.6em; }
h3 { font-size: 1.12rem; margin-top: 1.2em; }
p { margin: 0 0 .9em; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

/* top bar */
.top { position: sticky; top: 0; z-index: 10; display: flex; align-items: center; gap: 14px; padding: 10px 16px; padding-top: max(10px, env(safe-area-inset-top)); background: color-mix(in srgb, var(--bg) 92%, transparent); backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); }
.brand { display: flex; align-items: center; gap: 8px; text-decoration: none; color: var(--ink); font-family: var(--font-head); font-size: 1.15rem; white-space: nowrap; }
.brand svg { width: 28px; height: 28px; }
.brand b { color: var(--accent); }
.search { flex: 1; max-width: 520px; }
.search input { width: 100%; padding: 9px 14px; border-radius: 999px; border: 1px solid var(--line); background: var(--paper); color: var(--ink); font: 1rem var(--font-ui); }
.search input:focus { outline: none; border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.topnav { display: flex; align-items: center; gap: 12px; margin-left: auto; }
.topnav a { text-decoration: none; color: var(--muted); font-weight: 700; font-size: .95rem; }
.topnav a:hover { color: var(--ink); }
.icon-btn { width: 36px; height: 36px; display: grid; place-items: center; border-radius: 10px; border: 1px solid var(--line); background: var(--paper); cursor: pointer; color: var(--muted); }
.icon-btn svg { width: 18px; height: 18px; }
@media (max-width: 680px) {
  .top { flex-wrap: wrap; gap: 8px 10px; }
  .search { order: 3; flex-basis: 100%; max-width: none; }
  .brand span { font-size: 1rem; }
  .brand svg { width: 24px; height: 24px; }
  .topnav { gap: 8px; }
  .topnav a { font-size: .86rem; }
  .icon-btn { width: 32px; height: 32px; }
}

main { width: 100%; max-width: 900px; margin: 0 auto; padding: 22px 16px 60px; outline: none; flex: 1; }
.foot { text-align: center; color: var(--muted); font-size: .8rem; padding: 18px 16px 28px; border-top: 1px solid var(--line); }
.muted { color: var(--muted); }
.eyebrow { font-size: .78rem; letter-spacing: .1em; text-transform: uppercase; font-weight: 700; color: var(--muted); margin-bottom: 6px; }
.crumbs { font-size: .9rem; color: var(--muted); margin-bottom: 12px; }
.crumbs a { color: var(--muted); }
.card { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); padding: 18px; }
.row { display: flex; gap: 10px; flex-wrap: wrap; align-items: center; }
.spacer { flex: 1; }
.chip { display: inline-flex; align-items: center; gap: 6px; font-size: .78rem; font-weight: 700; padding: 2px 10px; border-radius: 999px; border: 1px solid var(--line); color: var(--muted); background: var(--paper); white-space: nowrap; }
.chip.strand { color: var(--sc); border-color: color-mix(in srgb, var(--sc) 45%, transparent); background: color-mix(in srgb, var(--sc) 10%, var(--paper)); }
.btn { display: inline-flex; align-items: center; justify-content: center; gap: 6px; padding: 9px 16px; border-radius: 10px; border: 1px solid var(--line); background: var(--paper); color: var(--ink); font-weight: 700; font-size: .95rem; cursor: pointer; text-decoration: none; }
.btn:hover { border-color: var(--accent); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--paper); }
.btn.small { padding: 5px 11px; font-size: .85rem; border-radius: 8px; }
.btn.on { background: var(--accent-soft); border-color: var(--accent); color: var(--accent); }
.btn:disabled { opacity: .45; cursor: not-allowed; }

/* home */
.hero p { color: var(--muted); max-width: 62ch; font-size: 1.05rem; }
.year-pick { display: inline-flex; border: 1px solid var(--line); border-radius: 999px; overflow: hidden; background: var(--paper); }
.year-pick button { border: 0; background: transparent; padding: 6px 14px; cursor: pointer; font-weight: 700; color: var(--muted); }
.year-pick button.on { background: var(--accent); color: var(--paper); }
.strands { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 14px; margin-top: 18px; }
.strand-card { border-top: 5px solid var(--sc); }
.strand-card h3 { margin-top: 0; }
.strand-card ul { list-style: none; padding: 0; margin: 10px 0 0; }
.strand-card li { padding: 5px 0; border-top: 1px dashed var(--line); font-size: .95rem; display: flex; gap: 8px; align-items: baseline; }
.strand-card li a { text-decoration: none; color: var(--ink); flex: 1; }
.strand-card li a:hover { color: var(--accent); text-decoration: underline; }
.yr { font-size: .7rem; color: var(--muted); white-space: nowrap; }
.tiles { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: 12px; margin-top: 14px; }
.tile-link { text-decoration: none; color: var(--ink); display: block; }
.tile-link:hover { border-color: var(--accent); }
.tile-link b { display: block; font-family: var(--font-head); font-size: 1.05rem; }
.tile-link span { color: var(--muted); font-size: .88rem; }
.results .r { display: block; padding: 12px 14px; border: 1px solid var(--line); border-radius: 12px; background: var(--paper); margin-bottom: 8px; text-decoration: none; color: var(--ink); }
.results .r:hover { border-color: var(--accent); }
.results .r b { font-family: var(--font-head); }
.results .r small { display: block; color: var(--muted); }

/* topic page */
.topic-head .summary { font-size: 1.08rem; color: var(--muted); max-width: 62ch; }
.toc { position: sticky; top: 62px; z-index: 5; display: flex; gap: 6px; overflow-x: auto; padding: 8px 0; margin: 10px 0 6px; background: var(--bg); border-bottom: 1px solid var(--line); scrollbar-width: none; }
.toc a { flex: none; text-decoration: none; font-weight: 700; font-size: .88rem; color: var(--muted); padding: 5px 12px; border-radius: 999px; border: 1px solid var(--line); background: var(--paper); }
.toc a:hover { color: var(--accent); border-color: var(--accent); }
@media (max-width: 680px) { .toc { top: 104px; } }
section.block { scroll-margin-top: 120px; }
.learn-sec { margin-bottom: 18px; }
.learn-sec h3 { color: var(--sc, var(--accent)); }
.callout { border: 1px solid color-mix(in srgb, var(--accent) 40%, transparent); background: var(--accent-soft); border-radius: 12px; padding: 12px 14px; margin: 12px 0; }
.callout.warn { border-color: var(--warn-line); background: var(--warn-bg); }
.mathblock { font-family: var(--font-math); font-size: 1.45rem; text-align: center; margin: 12px 0; }
.vocab-list { display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr)); gap: 10px; }
.vocab-list div { background: var(--paper); border: 1px solid var(--line); border-radius: 12px; padding: 10px 12px; }
.vocab-list dt { font-weight: 700; }
.vocab-list dd { margin: 2px 0 0; color: var(--muted); font-size: .95rem; }
.mistakes { padding: 0; list-style: none; }
.mistakes li { border-left: 4px solid var(--bad); background: var(--paper); border-radius: 0 12px 12px 0; padding: 10px 14px; margin-bottom: 10px; }
.mistakes li::before { content: "✗ "; color: var(--bad); font-weight: 700; }
.related a { margin-right: 8px; }
.prevnext { display: flex; justify-content: space-between; gap: 10px; margin-top: 26px; flex-wrap: wrap; }

/* math */
.m { font-family: var(--font-math); font-size: 1.1em; white-space: nowrap; }
.m i { font-style: italic; padding: 0 .02em; }
.op { margin: 0 .22em; display: inline-block; }
@media (max-width: 680px) { main .m { white-space: normal; } main .m .frac, main .m sup, main .m sub { white-space: nowrap; } .mathblock { overflow-wrap: anywhere; } }
.m sup, .m sub { font-size: .7em; line-height: 0; }
.frac { display: inline-flex; flex-direction: column; vertical-align: middle; text-align: center; margin: 0 .12em; font-size: .92em; }
.frac > span:first-child { border-bottom: 1px solid currentColor; padding: 0 .2em; }
.frac > span:last-child { padding: 0 .2em; }
table.data, table.vocab { border-collapse: collapse; margin: 10px 0; max-width: 100%; }
table.data td, table.data th, table.vocab td, table.vocab th { border: 1px solid var(--line); padding: 6px 10px; text-align: center; font-variant-numeric: tabular-nums; }
table.vocab td, table.vocab th { text-align: left; }
table.data th, table.vocab th { color: var(--muted); background: color-mix(in srgb, var(--line) 35%, transparent); }
.anatomy { display: flex; justify-content: center; gap: 4px; flex-wrap: wrap; margin: 18px 0 8px; font-family: var(--font-math); font-size: 1.8rem; }
.anatomy > span { display: flex; flex-direction: column; align-items: center; }
.anatomy small { font-family: var(--font-ui); font-size: .72rem; color: var(--muted); border-top: 2px solid var(--c, var(--accent)); padding-top: 4px; margin-top: 4px; white-space: nowrap; }
.scroll-x { overflow-x: auto; }

/* worked example */
.example { background: var(--paper); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; }
.ex-bar { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; margin-bottom: 12px; }
.ex-prompt { font-size: 1.08rem; margin-bottom: 10px; }
.ex-visual { margin: 10px 0; overflow-x: auto; }
.ex-visual svg { max-width: 100%; height: auto; }
.ex-kind { font-size: .75rem; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--c, var(--muted)); }
.solution-box { margin-top: 12px; border-top: 1px dashed var(--line); padding-top: 12px; }
.steps { margin: 8px 0; padding-left: 0; list-style: none; counter-reset: s; }
.steps li { counter-increment: s; padding: 6px 10px 6px 40px; position: relative; border-left: 3px solid var(--line); margin-bottom: 4px; }
.steps li::before { content: counter(s); position: absolute; left: 10px; top: 6px; width: 20px; height: 20px; border-radius: 50%; background: var(--accent-soft); color: var(--accent); font-size: .75rem; font-weight: 700; display: grid; place-items: center; }
.steps li.hid { display: none; }
.solution-box ol:not(.steps) li.hid, .solution-box .hid { display: none; }
.ans-line { margin-top: 8px; font-weight: 700; }

/* try-it */
.answer-row { display: flex; gap: 10px; flex-wrap: wrap; align-items: center; }
input.ans { flex: 1; min-width: 140px; padding: 10px 12px; border-radius: 10px; border: 1px solid var(--line); background: var(--bg); color: var(--ink); font: 1.2rem var(--font-math); }
input.ans:focus { outline: none; border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.multi-f { display: flex; align-items: center; gap: 8px; flex: 1; min-width: 130px; font: 1.15rem var(--font-math); }
.keypad { display: flex; gap: 6px; flex-wrap: wrap; margin-top: 8px; }
.keypad button { min-width: 40px; height: 36px; border-radius: 8px; border: 1px solid var(--line); background: var(--bg); cursor: pointer; font: 1.05rem var(--font-math); }
.options { display: grid; gap: 8px; grid-template-columns: repeat(auto-fit, minmax(170px, 1fr)); }
.opt { text-align: left; padding: 10px 14px; border-radius: 10px; border: 1px solid var(--line); background: var(--bg); cursor: pointer; }
.opt:hover:not(:disabled) { border-color: var(--accent); }
.opt.right { border-color: var(--good); background: color-mix(in srgb, var(--good) 12%, var(--paper)); }
.opt.wrong { border-color: var(--bad); background: color-mix(in srgb, var(--bad) 12%, var(--paper)); }
.order-ans { min-height: 50px; border: 1px dashed var(--line); border-radius: 10px; padding: 6px; margin-bottom: 8px; display: flex; flex-direction: column; gap: 6px; }
.order-ans:empty::before { content: "Tap the steps below in order"; color: var(--muted); font-size: .9rem; padding: 6px; }
.order-pool { display: flex; flex-direction: column; gap: 6px; }
.order-item { text-align: left; padding: 8px 12px; border-radius: 8px; border: 1px solid var(--line); background: var(--bg); cursor: pointer; }
.fb { margin-top: 12px; border-radius: 10px; padding: 10px 14px; }
.fb.good { background: color-mix(in srgb, var(--good) 12%, var(--paper)); border: 1px solid var(--good); }
.fb.bad { background: color-mix(in srgb, var(--bad) 10%, var(--paper)); border: 1px solid var(--bad); }
.fb.meh { background: var(--warn-bg); border: 1px solid var(--warn-line); }
.hint { margin-top: 8px; color: var(--gold); }

/* guides & glossary */
.guide-list { display: grid; gap: 12px; }
.glossary dl { margin: 0; }
.glossary .g { display: grid; grid-template-columns: minmax(120px, 220px) 1fr; gap: 10px; padding: 8px 0; border-bottom: 1px solid var(--line); }
.glossary dt { font-weight: 700; }
.glossary dd { margin: 0; color: var(--muted); }
.glossary dd a { font-size: .85rem; margin-right: 8px; }
.letters { display: flex; flex-wrap: wrap; gap: 4px; margin: 10px 0; }
.letters a { text-decoration: none; width: 30px; text-align: center; border: 1px solid var(--line); border-radius: 6px; background: var(--paper); font-weight: 700; }
@media (max-width: 560px) { .glossary .g { grid-template-columns: 1fr; gap: 2px; } }

/* ===== manipulative kit: always on a dark "screen", matching Algebra Odyssey ===== */
.manip { --ink: #eaf0ff; --muted: #9ba8d6; --dim: #5d6894; --line: rgba(140, 160, 255, .22); --cyan: #74f0ff; --gold: #ffd36e; --good: #4ff0a8; --bad: #ff6382;
  --A: #5aa9ff; --B: #b48cff; --C: #3fe0b0; --D: #ff9d5c; --accent: #74f0ff; --bg: #0f1630; --panel: rgba(18, 26, 58, .78); --panel-solid: #121a3a; --font-head: "Atkinson Hyperlegible", sans-serif;
  color: var(--ink); background: #0f1630; border-radius: 14px; padding: 14px; margin: 14px 0; border: 1px solid #26305a; overflow-x: auto; }
.manip .manip-title { font-weight: 700; font-size: .8rem; text-transform: uppercase; letter-spacing: .1em; color: var(--cyan); margin-bottom: 10px; }
.manip .seg { display: flex; gap: 6px; flex-wrap: wrap; margin-bottom: 10px; }
.manip .seg button { padding: 6px 12px; border-radius: 999px; border: 1px solid var(--line); background: transparent; cursor: pointer; font: 1rem var(--font-math); color: var(--ink); }
.manip .seg button.on { background: rgba(116, 240, 255, .15); border-color: var(--cyan); }
.manip .slider-row { display: flex; align-items: center; gap: 12px; margin: 8px 0; }
.manip .slider-row label { min-width: 70px; font-family: var(--font-math); font-size: 1.1rem; }
.manip .slider-row input[type=range] { flex: 1; accent-color: var(--cyan); min-width: 0; }
.manip .slider-row output { min-width: 34px; text-align: right; font: 1.15rem var(--font-math); color: var(--gold); }
.manip .btn { background: #121a3a; color: var(--ink); border-color: var(--line); }
.manip .btn.primary { background: linear-gradient(135deg, #2bb8d6, #5a6cff); border-color: transparent; color: #fff; }
.manip .btn.ghost { background: transparent; }
.manip table.data td, .manip table.data th { border-color: var(--line); }
.manip table.data th { background: rgba(255,255,255,.04); color: var(--muted); }
.manip .callout { background: rgba(116, 240, 255, .06); border-color: rgba(116, 240, 255, .35); }
.manip .machine { display: grid; grid-template-columns: 1fr auto 1fr; align-items: center; gap: 10px; text-align: center; margin: 12px 0; }
.manip .machine .io { border: 1px solid var(--line); border-radius: 12px; padding: 10px; font: 1.5rem var(--font-math); }
.manip .machine .io small { display: block; font: .7rem var(--font-ui); color: var(--muted); text-transform: uppercase; letter-spacing: .1em; }
.manip .machine .box { padding: 12px 14px; border-radius: 14px; background: linear-gradient(135deg, #2a3a7a, #172050); border: 1px solid var(--cyan); font: 1.25rem var(--font-math); }
.manip .machine .arrow { color: var(--cyan); }
.manip .history, .manip .readout { font-size: .95rem; color: var(--muted); margin-top: 6px; }
.manip .readout { color: var(--ink); }
.manip .tray { min-height: 120px; display: flex; flex-wrap: wrap; gap: 8px; align-items: flex-end; padding: 12px; border-radius: 12px; border: 1px dashed var(--line); margin: 10px 0; }
.manip .tile { border: 0; cursor: pointer; border-radius: 6px; display: grid; place-items: center; font: italic 1rem var(--font-math); color: #071022; transition: transform .2s, opacity .3s; }
.manip .tile.x { width: 34px; height: 96px; } .manip .tile.one { width: 34px; height: 34px; } .manip .tile.x2 { width: 96px; height: 96px; }
.manip .tile.pos { background: linear-gradient(135deg, #74f0ff, #3aa5d6); } .manip .tile.neg { background: linear-gradient(135deg, #ff8aa3, #d6405f); }
.manip .tile.pair { outline: 3px solid var(--gold); } .manip .tile.gone { transform: scale(0); opacity: 0; }
.manip .area-svg text { font-family: var(--font-math); fill: var(--ink); }
.manip .sticks-table { max-height: 220px; overflow: auto; }
.manip .row { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }
.manip .spacer { flex: 1; }
.manip .muted { color: var(--muted); }

@media print {
  .top, .toc, .ex-bar, .foot, .manip, .prevnext { display: none !important; }
  body { background: #fff; color: #000; }
  .steps li.hid, .solution-box .hid { display: list-item !important; }
}