:root {
  color-scheme: light dark;
  --bg: #f7f7f5;
  --card: #ffffff;
  --ink: #1b1b1a;
  --muted: #6b6b66;
  --line: #d9d9d4;
  --accent: #2f6f5e;
  --accent-soft: #e3efeb;
  --grid: #ececea;
  --axis: #b4b4ae;
  --curve: #2f6f5e;
  --student: #c2622a;
  --answer: #7a4fb0;
  --right: #2f6f5e;
  --wrong: #b03a34;
  --right-soft: #e3efeb;
  --wrong-soft: #f6e5e4;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #17181a;
    --card: #202225;
    --ink: #ececeb;
    --muted: #9b9b96;
    --line: #34363a;
    --accent: #6fbfa6;
    --accent-soft: #24352f;
    --grid: #2a2c30;
    --axis: #55585d;
    --curve: #6fbfa6;
    --student: #e0925a;
    --answer: #b08ad8;
    --right: #6fbfa6;
    --wrong: #e0837c;
    --right-soft: #22332d;
    --wrong-soft: #352523;
  }
}

* { box-sizing: border-box; }

body {
  margin: 0;
  padding: 0 1.5rem 3rem;
  background: var(--bg);
  color: var(--ink);
  font: 15px/1.55 ui-sans-serif, system-ui, -apple-system, "Segoe UI", sans-serif;
}

main { max-width: 1180px; margin: 0 auto; }

.hero { padding: 2.5rem 0 1.5rem; }
.eyebrow {
  margin: 0 0 .35rem;
  font-size: .74rem;
  letter-spacing: .09em;
  text-transform: uppercase;
  color: var(--muted);
}
h1 { margin: 0 0 .5rem; font-size: 1.9rem; letter-spacing: -.02em; }
h2 { margin: 0; font-size: 1.05rem; }
.lede { margin: 0; max-width: 46rem; color: var(--muted); }

.card {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: 12px;
  padding: 1.15rem 1.25rem;
}

.panels {
  display: grid;
  grid-template-columns: minmax(0, 5fr) minmax(0, 7fr);
  gap: 1.1rem;
  align-items: start;
}

@media (max-width: 900px) {
  .panels { grid-template-columns: minmax(0, 1fr); }
}

.panel-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 1rem;
  padding-bottom: .85rem;
  margin-bottom: .95rem;
  border-bottom: 1px solid var(--line);
}
.counter { font-size: .8rem; color: var(--muted); font-variant-numeric: tabular-nums; }

.problem-panel { display: flex; flex-direction: column; }

.statement {
  margin: .2rem 0 0;
  font-size: 1.15rem;
  overflow-x: auto;
  overflow-y: hidden;
  padding: .15rem 0;
}
.statement.point { margin-top: .5rem; color: var(--muted); font-size: 1rem; }

#answer-form { margin-top: 1.4rem; }
label {
  display: block;
  font-size: .84rem;
  color: var(--muted);
  margin-bottom: .4rem;
}
.entry { display: flex; gap: .5rem; }
input {
  flex: 1;
  min-width: 0;
  font: inherit;
  font-size: 1rem;
  padding: .5rem .65rem;
  color: var(--ink);
  background: var(--bg);
  border: 1px solid var(--line);
  border-radius: 8px;
}
input:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 1px;
  border-color: transparent;
}

button {
  font: inherit;
  font-size: .9rem;
  padding: .5rem .95rem;
  border-radius: 8px;
  border: 1px solid transparent;
  background: var(--accent);
  color: var(--card);
  cursor: pointer;
}
button:hover { filter: brightness(1.08); }
button.ghost {
  background: transparent;
  color: var(--ink);
  border-color: var(--line);
}
button.ghost:hover { background: var(--accent-soft); }

/* "Previous" is disabled on the first problem, since a random order has
   nothing behind it yet.  It must not light up under the cursor. */
button:disabled { opacity: .4; cursor: default; }
button:disabled:hover { filter: none; }
button.ghost:disabled:hover { background: transparent; }

.hint { margin: .55rem 0 0; font-size: .8rem; color: var(--muted); }
code {
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  font-size: .95em;
  background: var(--bg);
  border: 1px solid var(--line);
  border-radius: 4px;
  padding: 0 .28em;
}

.feedback {
  margin-top: 1rem;
  padding: .7rem .85rem;
  border-radius: 8px;
  font-size: .92rem;
  border: 1px solid transparent;
}
.feedback.right { background: var(--right-soft); border-color: var(--right); color: var(--right); }
.feedback.wrong { background: var(--wrong-soft); border-color: var(--wrong); color: var(--wrong); }
.feedback.note { background: var(--bg); border-color: var(--line); color: var(--muted); }

.answer-reveal {
  margin-top: .85rem;
  padding: .75rem .85rem;
  border: 1px dashed var(--answer);
  border-radius: 8px;
  overflow-x: auto;
}
.answer-reveal .label {
  font-size: .74rem;
  letter-spacing: .09em;
  text-transform: uppercase;
  color: var(--muted);
  display: block;
  margin-bottom: .35rem;
}

.actions {
  display: flex;
  gap: .5rem;
  margin-top: auto;
  padding-top: 1.4rem;
}
.actions #show { margin-left: auto; }

canvas {
  display: block;
  width: 100%;
  aspect-ratio: 4 / 3;
  max-height: 62vh;
  cursor: grab;
  /* A finger on the graph drags the graph rather than scrolling the page. */
  touch-action: none;
}
canvas.dragging { cursor: grabbing; }

.zoom {
  display: flex;
  align-items: center;
  gap: .7rem;
  margin-top: .9rem;
}
.zoom label {
  font-size: .72rem;
  letter-spacing: .09em;
  text-transform: uppercase;
  color: var(--muted);
  margin: 0;
}
.zoom input[type="range"] {
  flex: 1;
  min-width: 0;
  accent-color: var(--accent);
  cursor: pointer;
}
.zoom #zoom-readout { min-width: 4.2rem; text-align: right; }

button.small {
  padding: .3rem .6rem;
  font-size: .78rem;
}

.zoom-note #zoom-window {
  color: var(--ink);
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
}

.legend {
  display: flex;
  flex-wrap: wrap;
  gap: 1.1rem;
  margin: .85rem 0 0;
  padding: 0;
  list-style: none;
  font-size: .82rem;
  color: var(--muted);
}
.legend li { display: flex; align-items: center; gap: .45rem; }
.swatch { width: 20px; height: 3px; border-radius: 2px; display: inline-block; }
.swatch.curve { background: var(--curve); }
.swatch.student { background: var(--student); }
.swatch.answer {
  background: repeating-linear-gradient(90deg, var(--answer) 0 5px, transparent 5px 9px);
}

.katex { font-size: 1.05em; }