:root {
  --paper: #e9f0e6;      /* engineering-pad green */
  --grid: #d3e0cf;
  --sheet: #f7faf5;
  --ink: #243a5e;        /* pen navy */
  --pencil: #5d6a63;
  --rule: #b9cbb4;
  --margin: #c96f6f;     /* red margin line */
  --good: #23704a;
  --good-bg: #dcefe1;
  --bad: #a3322b;
  --bad-bg: #f6e0dd;
  --focus: #e0a800;
  --radius: 6px;
  --f-head: 'Bricolage Grotesque', system-ui, sans-serif;
  --f-body: 'Atkinson Hyperlegible', system-ui, sans-serif;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --paper: #17201b; --grid: #202c25; --sheet: #1d2721; --ink: #dce7f5; --pencil: #9fb0a6;
    --rule: #34463b; --margin: #b26060; --good: #7fd3a2; --good-bg: #1f3a2a; --bad: #f09a92; --bad-bg: #3e2220;
  }
}
:root[data-theme="dark"] {
  --paper: #17201b; --grid: #202c25; --sheet: #1d2721; --ink: #dce7f5; --pencil: #9fb0a6;
  --rule: #34463b; --margin: #b26060; --good: #7fd3a2; --good-bg: #1f3a2a; --bad: #f09a92; --bad-bg: #3e2220;
}

* { box-sizing: border-box; }
[hidden] { display: none !important; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  font: 17px/1.55 var(--f-body);
  color: var(--ink);
  background-color: var(--paper);
  background-image:
    linear-gradient(var(--grid) 1px, transparent 1px),
    linear-gradient(90deg, var(--grid) 1px, transparent 1px);
  background-size: 24px 24px;
  min-height: 100vh;
}
h1, h2, h3 { font-family: var(--f-head); line-height: 1.15; margin: 0 0 .5em; letter-spacing: -.01em; }
h1 { font-size: 2.1rem; font-weight: 700; }
h2 { font-size: 1.35rem; font-weight: 700; margin-top: 1.6em; }
h3 { font-size: 1.08rem; font-weight: 500; }
p { margin: 0 0 1em; }
a { color: inherit; text-underline-offset: 3px; }
:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; }
.muted { color: var(--pencil); }
.small { font-size: .9rem; }

/* top bar */
.topbar {
  display: flex; align-items: center; justify-content: space-between; gap: 16px;
  padding: 12px max(16px, calc((100vw - 980px) / 2));
  background: var(--sheet); border-bottom: 1px solid var(--rule);
  position: sticky; top: 0; z-index: 5;
}
.brand { display: flex; align-items: center; gap: 10px; font: 700 1.1rem var(--f-head); text-decoration: none; }
.brand svg { width: 26px; height: 26px; fill: none; stroke: var(--ink); stroke-width: 2.6; stroke-linecap: round; }
#nav { display: flex; align-items: center; gap: 4px; flex-wrap: wrap; justify-content: flex-end; }
#nav a, #nav button {
  font: inherit; font-size: .95rem; color: var(--ink); background: none; border: 0; cursor: pointer;
  padding: 6px 10px; border-radius: var(--radius); text-decoration: none;
}
#nav a[aria-current="page"] { background: var(--paper); }
#nav a:hover, #nav button:hover { text-decoration: underline; }
#nav .who { color: var(--pencil); font-size: .9rem; padding: 0 6px; }

main { max-width: 980px; margin: 0 auto; padding: 32px 16px 80px; outline: none; }
.narrow { max-width: 460px; margin: 0 auto; }
.crumbs { font-size: .92rem; color: var(--pencil); margin-bottom: 6px; }
.head-row { display: flex; align-items: flex-end; justify-content: space-between; gap: 16px; flex-wrap: wrap; margin-bottom: 8px; }
.head-row h1 { margin: 0; }
.actions { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }

/* surfaces: plain sheets of paper, no shadows */
.sheet { background: var(--sheet); border: 1px solid var(--rule); border-radius: var(--radius); padding: 20px; }
.stack > * + * { margin-top: 12px; }

/* buttons + forms */
button, .btn {
  font: 700 .95rem var(--f-body); cursor: pointer; text-decoration: none; display: inline-flex; align-items: center; gap: 6px;
  padding: 9px 16px; border-radius: var(--radius); border: 2px solid var(--ink); background: var(--ink); color: var(--sheet);
}
button.ghost, .btn.ghost { background: transparent; color: var(--ink); }
button.danger { background: transparent; color: var(--bad); border-color: var(--bad); }
button.link { background: none; border: 0; padding: 0; color: var(--ink); text-decoration: underline; font-weight: 400; }
button:disabled { opacity: .5; cursor: not-allowed; }
label { display: block; font-weight: 700; font-size: .95rem; margin-bottom: 4px; }
.field { margin-bottom: 16px; }
.hint { font-weight: 400; color: var(--pencil); font-size: .88rem; }
input, select, textarea {
  font: inherit; color: var(--ink); background: var(--sheet); width: 100%;
  border: 1.5px solid var(--rule); border-radius: var(--radius); padding: 9px 11px;
}
input:focus, select:focus, textarea:focus { border-color: var(--ink); outline: none; box-shadow: 0 0 0 3px color-mix(in srgb, var(--focus) 45%, transparent); }
textarea { min-height: 120px; resize: vertical; line-height: 1.5; }
.row { display: grid; gap: 12px; grid-template-columns: repeat(auto-fit, minmax(150px, 1fr)); }
.seg { display: flex; border: 1.5px solid var(--rule); border-radius: var(--radius); overflow: hidden; margin-bottom: 16px; }
.seg label { flex: 1; margin: 0; text-align: center; padding: 9px; cursor: pointer; font-weight: 400; }
.seg input { position: absolute; opacity: 0; width: 1px; }
.seg input:checked + span { font-weight: 700; }
.seg label:has(input:checked) { background: var(--ink); color: var(--sheet); }
.seg label:has(input:focus-visible) { outline: 3px solid var(--focus); outline-offset: -3px; }
.error { color: var(--bad); background: var(--bad-bg); padding: 10px 12px; border-radius: var(--radius); margin-bottom: 14px; }
.error:empty { display: none; }

/* lists + tables */
.list { list-style: none; margin: 0; padding: 0; border-top: 1px solid var(--rule); }
.list li { border-bottom: 1px solid var(--rule); padding: 14px 4px; display: flex; gap: 16px; justify-content: space-between; align-items: center; flex-wrap: wrap; }
.list .title { font: 500 1.08rem var(--f-head); }
.list a.title { text-decoration: none; }
.list a.title:hover { text-decoration: underline; }
.tag { display: inline-block; font-size: .8rem; padding: 1px 8px; border-radius: 99px; border: 1px solid var(--rule); color: var(--pencil); white-space: nowrap; }
.tag.good { color: var(--good); border-color: currentColor; }
.tag.bad { color: var(--bad); border-color: currentColor; }
.score { font: 700 1.05rem var(--f-head); white-space: nowrap; }
.table-wrap { overflow-x: auto; border: 1px solid var(--rule); border-radius: var(--radius); background: var(--sheet); }
table { border-collapse: collapse; width: 100%; font-size: .93rem; }
th, td { text-align: left; padding: 8px 12px; border-bottom: 1px solid var(--rule); white-space: nowrap; }
th { font-weight: 700; }
td.num, th.num { text-align: right; font-variant-numeric: tabular-nums; }
tr:last-child td { border-bottom: 0; }
.code { font: 700 1.6rem var(--f-head); letter-spacing: .12em; }
.empty { padding: 28px 20px; text-align: center; color: var(--pencil); border: 1.5px dashed var(--rule); border-radius: var(--radius); }

/* ---- the problem sheet: the one bold thing ---- */
.worksheet { background: var(--sheet); border: 1px solid var(--rule); border-radius: var(--radius); position: relative; overflow: hidden;
  background-image: linear-gradient(90deg, transparent 63px, var(--margin) 63px, var(--margin) 64px, transparent 64px); }
.problem { display: grid; grid-template-columns: 64px 1fr; border-bottom: 1px solid var(--rule); }
.problem:last-child { border-bottom: 0; }
.problem .num {
  font: 700 2rem/1 var(--f-head); text-align: center; padding-top: 22px; color: var(--ink);
}
.problem .body { padding: 20px 22px 22px; min-width: 0; }
.problem .meta { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; margin-bottom: 8px; font-size: .88rem; color: var(--pencil); }
.problem .text { font-size: 1.08rem; white-space: pre-wrap; margin-bottom: 14px; max-width: 68ch; }
.answer-row { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; }
.answer-row input { width: 200px; font-variant-numeric: tabular-nums; }
.answer-row .units { font-weight: 700; }
.choices { display: grid; gap: 6px; margin-bottom: 12px; max-width: 60ch; }
.choices label { display: flex; gap: 10px; align-items: flex-start; font-weight: 400; padding: 8px 10px; border: 1.5px solid var(--rule); border-radius: var(--radius); cursor: pointer; margin: 0; }
.choices label:has(input:checked) { border-color: var(--ink); }
.choices input { width: auto; margin-top: 5px; accent-color: var(--ink); }
.feedback { margin-top: 10px; font-weight: 700; padding: 6px 10px; border-radius: var(--radius); display: inline-block; }
.feedback.good { color: var(--good); background: var(--good-bg); }
.feedback.bad { color: var(--bad); background: var(--bad-bg); }
.problem.solved .num { color: var(--good); }
.problem.solved .num::after { content: '✓'; display: block; font-size: 1.3rem; margin-top: 4px; }
.key { margin-top: 10px; font-size: .92rem; color: var(--pencil); }
.key b { color: var(--good); }
.flash { animation: flash .6s ease-out; }
@keyframes flash { from { background: var(--good-bg); } to { background: transparent; } }
@keyframes shake { 25% { transform: translateX(-4px); } 75% { transform: translateX(4px); } }
.shake { animation: shake .25s 2; }

/* homework builder */
.picker { display: grid; gap: 16px; grid-template-columns: 1fr 1fr; align-items: start; }
.picker .pane { max-height: 70vh; overflow: auto; }
.pick { padding: 10px 12px; border-bottom: 1px solid var(--rule); display: flex; gap: 10px; align-items: flex-start; }
.pick:last-child { border-bottom: 0; }
.pick p { margin: 0; font-size: .93rem; }
.pick .grow { flex: 1; min-width: 0; }
.pick input[type=number] { width: 64px; padding: 5px 6px; }
.pick button { padding: 4px 10px; font-size: .85rem; }
.pick-tools { display: flex; gap: 6px; align-items: center; margin-top: 8px; flex-wrap: wrap; }
.pick-tools label { display: flex; gap: 6px; align-items: center; margin-right: auto; }
.choice-edit { display: flex; gap: 8px; align-items: center; margin-bottom: 8px; }
.choice-edit input[type=radio] { width: auto; accent-color: var(--ink); }

#toast { position: fixed; bottom: 20px; left: 50%; transform: translateX(-50%) translateY(20px); opacity: 0; transition: .2s;
  background: var(--ink); color: var(--sheet); padding: 10px 18px; border-radius: var(--radius); pointer-events: none; z-index: 10; max-width: calc(100vw - 32px); }
#toast.show { opacity: 1; transform: translateX(-50%); }

@media (max-width: 720px) {
  body { font-size: 16px; }
  h1 { font-size: 1.7rem; }
  .picker { grid-template-columns: 1fr; }
  .picker .pane { max-height: none; }
  .worksheet { background-image: linear-gradient(90deg, transparent 43px, var(--margin) 43px, var(--margin) 44px, transparent 44px); }
  .problem { grid-template-columns: 44px 1fr; }
  .problem .num { font-size: 1.5rem; }
  .problem .body { padding: 16px 14px 18px; }
  .answer-row input { width: 100%; flex: 1 1 140px; }
  #nav .who { display: none; }
  .topbar { padding-top: 8px; padding-bottom: 8px; }
}
@media (prefers-reduced-motion: reduce) { *, *::before, *::after { animation: none !important; transition: none !important; } }

/* ---- symbolic answers ---- */
code { font-family: ui-monospace, 'SFMono-Regular', Menlo, monospace; font-size: .88em; background: var(--paper); padding: 1px 5px; border-radius: 4px; }
input.expr { font-family: ui-monospace, 'SFMono-Regular', Menlo, monospace; }
.answer-row input.expr { width: min(420px, 100%); flex: 1 1 260px; }
.expr-preview { min-height: 1.6em; margin: 8px 0 4px; font-size: 1.05rem; }
.expr-preview .muted { font-size: .85rem; margin-right: 6px; }
.expr-preview.bad { color: var(--bad); font-size: .9rem; }
.problem .hint { margin: 4px 0 0; }

/* ---- written answers ---- */
.written-input { max-width: 68ch; min-height: 90px; margin-bottom: 10px; }
.written { white-space: pre-wrap; max-width: 60ch; }

/* ---- free-body diagrams ---- */
.fbd { display: grid; grid-template-columns: minmax(220px, 300px) 1fr; gap: 18px; align-items: start; margin-bottom: 12px; }
.fbd-canvas { background: var(--paper); border: 1.5px solid var(--rule); border-radius: var(--radius); touch-action: none; cursor: crosshair; user-select: none; }
.fbd-svg { display: block; width: 100%; height: auto; }
.fbd-axis { stroke: var(--rule); stroke-width: 1; stroke-dasharray: 4 5; }
.fbd-ground { stroke: var(--pencil); stroke-width: 2.5; }
.fbd-surface line:not(.fbd-ground) { stroke: var(--pencil); stroke-width: 1.2; }
.fbd-body { fill: var(--sheet); stroke: var(--ink); stroke-width: 2; }
.fbd-dot { fill: var(--ink); }
.fbd-note { fill: var(--pencil); font: 12px var(--f-body); }
.fbd-arrow line { stroke: var(--ink); stroke-width: 3.2; stroke-linecap: round; }
.fbd-arrow polygon { fill: var(--ink); }
.fbd-arrow text { fill: var(--ink); font: italic 700 17px var(--f-body); }
.fbd-arrow.sel line { stroke: var(--margin); }
.fbd-arrow.sel polygon, .fbd-arrow.sel text { fill: var(--margin); }
.fbd-handle { fill: transparent; stroke: transparent; cursor: grab; }
.fbd-arrow.sel .fbd-handle { stroke: var(--margin); stroke-width: 1.5; stroke-dasharray: 3 3; }
.fbd-add { display: flex; gap: 8px; margin-bottom: 10px; }
.fbd-add select { flex: 1; min-width: 0; }
.fbd-list { list-style: none; padding: 0; margin: 0 0 8px; display: grid; gap: 6px; }
.fbd-list li { display: flex; gap: 6px; align-items: center; padding: 6px; border: 1.5px solid var(--rule); border-radius: var(--radius); }
.fbd-list li.sel { border-color: var(--margin); }
.fbd-list li select { flex: 1; min-width: 0; padding: 6px 8px; }
.fbd-angle { display: flex; align-items: center; gap: 2px; }
.fbd-angle input { width: 72px; padding: 6px; }
.fbd-list button { padding: 5px 10px; font-size: .85rem; }
.fbd-static { display: flex; gap: 14px; align-items: center; flex-wrap: wrap; margin-top: 6px; }
.fbd-static svg { background: var(--paper); border: 1px solid var(--rule); border-radius: var(--radius); }
.fbd-static ul { margin: 0; padding-left: 18px; }

/* ---- review page ---- */
.list.review li { align-items: flex-start; flex-wrap: nowrap; }
.list.review .who-col { flex: 0 0 220px; }
.list.review .ans-col { flex: 1; min-width: 0; }
.key ul { margin: 4px 0 6px; padding-left: 20px; }

@media (max-width: 720px) {
  .fbd { grid-template-columns: 1fr; }
  .fbd-canvas { max-width: 320px; }
  .list.review li { flex-wrap: wrap; }
  .list.review .who-col { flex-basis: 100%; }
}
.list .fbd-static li { display: list-item; border: 0; padding: 1px 0; }
.fbd-static svg { flex: 0 0 auto; }
@media (max-width: 720px) {
  .fbd-list li { flex-wrap: wrap; }
  .fbd-list li select { flex-basis: 100%; }
  .fbd-list li button { margin-left: auto; }
}

/* ---- instructor student mode ---- */
.preview-bar { display: flex; gap: 14px 20px; align-items: center; justify-content: space-between; flex-wrap: wrap;
  background: color-mix(in srgb, var(--focus) 18%, var(--sheet)); border: 1.5px solid var(--focus); border-radius: var(--radius);
  padding: 12px 16px; margin: 4px 0 18px; }
.preview-bar > div:first-child { flex: 1 1 380px; max-width: 72ch; font-size: .95rem; }