* { box-sizing: border-box; }
:root {
  --bg: #f7f9f8; --surface: #ffffff; --surface-2: #eef3f1;
  --ink: #1c2b27; --ink-soft: #4d5f5a; --ink-faint: #7d8c87;
  --line: #d8e2de; --accent: #0e7c66; --accent-soft: #e2f0ec;
  --amber: #a06b12; --amber-soft: #f6ecd9; --danger: #a4392f; --danger-soft: #f7e4e2;
}
body {
  margin: 0; background: var(--bg); color: var(--ink);
  font-family: "Segoe UI", system-ui, sans-serif; font-size: 14.5px; line-height: 1.5;
}
.hidden { display: none !important; }
button {
  background: var(--accent); color: #fff; border: 0; border-radius: 5px;
  padding: 8px 16px; font: inherit; font-weight: 600; cursor: pointer;
}
button:hover { filter: brightness(1.08); }
button.ghost { background: transparent; color: var(--ink-soft); border: 1px solid var(--line); }
button.small { padding: 4px 10px; font-size: 12.5px; }
button.danger { background: var(--danger); }
input, select, textarea {
  font: inherit; padding: 8px 10px; border: 1px solid var(--line); border-radius: 5px;
  background: var(--surface); color: var(--ink); width: 100%;
}
input:focus, textarea:focus, button:focus-visible { outline: 2px solid var(--accent); outline-offset: 1px; }
label { display: block; font-weight: 600; font-size: 13px; margin: 10px 0 2px; }
.muted { color: var(--ink-faint); }
.error { color: var(--danger); font-size: 13px; min-height: 18px; }

/* login */
.login { min-height: 100vh; display: grid; place-items: center; }
.login-card {
  background: var(--surface); border: 1px solid var(--line); border-radius: 8px;
  padding: 32px 36px; width: 340px;
}
.login-card h1 { margin: 0; font-size: 22px; color: var(--accent); }
.login-card p { margin-top: 2px; }
.login-card button { width: 100%; margin-top: 16px; }

/* shell */
.shell { display: flex; min-height: 100vh; }
.side {
  width: 210px; flex-shrink: 0; background: var(--surface);
  border-right: 1px solid var(--line); padding: 20px 14px;
  display: flex; flex-direction: column; gap: 6px;
}
.brand { font-weight: 700; color: var(--accent); font-size: 17px; margin-bottom: 14px; padding-left: 10px; }
.side nav { display: flex; flex-direction: column; gap: 2px; flex: 1; }
.side nav a {
  color: var(--ink-soft); text-decoration: none; padding: 8px 10px; border-radius: 5px; font-weight: 600;
}
.side nav a:hover { background: var(--surface-2); }
.side nav a.active { background: var(--accent-soft); color: var(--accent); }
main { flex: 1; padding: 28px 32px; max-width: 1080px; }
main h2 { margin: 0 0 4px; font-size: 21px; }
main .sub { color: var(--ink-faint); margin: 0 0 20px; }

/* cards & stats */
.stats { display: grid; grid-template-columns: repeat(auto-fill, minmax(170px, 1fr)); gap: 12px; margin: 18px 0; }
.stat {
  background: var(--surface); border: 1px solid var(--line); border-radius: 7px; padding: 14px 16px;
}
.stat b { display: block; font-size: 24px; font-variant-numeric: tabular-nums; }
.stat span { color: var(--ink-faint); font-size: 12.5px; }
.card {
  background: var(--surface); border: 1px solid var(--line); border-radius: 7px;
  padding: 18px 20px; margin: 14px 0;
}
.card h3 { margin: 0 0 10px; font-size: 15.5px; }
.row { display: flex; gap: 10px; align-items: end; flex-wrap: wrap; }
.row > * { flex: 1; min-width: 120px; }
.row > button { flex: 0 0 auto; }

/* tables */
table { border-collapse: collapse; width: 100%; font-size: 13.5px; }
th { text-align: left; font-size: 11px; text-transform: uppercase; letter-spacing: .07em;
     color: var(--ink-faint); padding: 6px 10px; border-bottom: 2px solid var(--line); }
td { padding: 8px 10px; border-bottom: 1px solid var(--line); vertical-align: top; }
.tablewrap { overflow-x: auto; }
.pill { display: inline-block; font-size: 11px; font-weight: 700; border-radius: 999px; padding: 1px 9px; }
.pill.green { background: var(--accent-soft); color: var(--accent); }
.pill.amber { background: var(--amber-soft); color: var(--amber); }
.pill.red { background: var(--danger-soft); color: var(--danger); }
.pill.grey { background: var(--surface-2); color: var(--ink-faint); }
code { background: var(--surface-2); border-radius: 4px; padding: 1px 6px; font-family: Consolas, monospace; font-size: .95em; }

/* content tree */
.subject { margin: 10px 0; }
.subject > summary { font-weight: 700; font-size: 15px; cursor: pointer; padding: 8px 4px; }
.chapter { margin: 6px 0 6px 18px; }
.chapter > summary { font-weight: 600; cursor: pointer; padding: 5px 4px; }
.section-row {
  display: flex; align-items: center; gap: 10px; margin-left: 22px;
  padding: 6px 8px; border-bottom: 1px solid var(--line);
}
.section-row .grow { flex: 1; }
textarea.import { font-family: Consolas, monospace; font-size: 13px; min-height: 260px; }
.toast {
  position: fixed; bottom: 20px; right: 20px; background: var(--ink); color: var(--bg);
  padding: 10px 18px; border-radius: 6px; font-weight: 600; opacity: 0; transition: opacity .2s;
}
.toast.show { opacity: 1; }

/* question viewer */
.qlist { margin: 4px 0 10px 40px; }
.qcard {
  background: var(--surface-2); border: 1px solid var(--line); border-radius: 6px;
  padding: 12px 16px; margin: 8px 0; font-size: 13.5px;
}
.qhead { display: flex; gap: 8px; align-items: center; margin-bottom: 6px; flex-wrap: wrap; }
.qtext { margin: 4px 0 8px; font-weight: 600; }
.choices { margin: 0 0 8px; padding-left: 26px; }
.choices li { margin: 2px 0; }
.choices li.right { color: var(--accent); font-weight: 700; }
.qexp { margin: 0; color: var(--ink-soft); border-top: 1px dashed var(--line); padding-top: 8px; }

/* Figures cropped from the textbook scans. Always on white — the scans are
   black ink on white paper, so a themed background would hide the axes. */
.qfig {
  display: block;
  max-width: 320px;
  max-height: 220px;
  margin: 8px 0;
  padding: 6px;
  background: #fff;
  border: 1px solid #d8dee9;
  border-radius: 6px;
}
.qfig-missing {
  margin: 8px 0;
  font-size: 13px;
  color: #b4232a;
}