:root {
  --bg: #f6f7f9;
  --panel: #ffffff;
  --ink: #16191d;
  --ink-soft: #5b6470;
  --line: #e2e6eb;
  --accent: #e8501e;
  --accent-soft: #fdf0eb;
  --ok: #1a7f4b;
  --ok-soft: #e8f6ee;
  --err: #c0392b;
  --err-soft: #fdecea;
  --code-bg: #1f232b;
  --radius: 10px;
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --mono: "JetBrains Mono", "SF Mono", Menlo, Consolas, monospace;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: var(--font);
  font-size: 16px;
  line-height: 1.6;
}

/* ---------------------------------------------------------------- layout */

.layout {
  display: grid;
  grid-template-columns: 280px minmax(0, 1fr);
  min-height: 100vh;
}

.sidebar {
  border-right: 1px solid var(--line);
  background: var(--panel);
  padding: 28px 20px;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.brand {
  font-weight: 700;
  font-size: 1.05rem;
  letter-spacing: -0.01em;
  margin-bottom: 2px;
}

.brand span { color: var(--accent); }

.brand-sub {
  color: var(--ink-soft);
  font-size: 0.82rem;
  margin-bottom: 26px;
}

.nav-group-title {
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--ink-soft);
  margin: 22px 0 8px;
  font-weight: 700;
}

.nav a {
  display: block;
  padding: 7px 10px;
  border-radius: 7px;
  color: var(--ink);
  text-decoration: none;
  font-size: 0.9rem;
  border-left: 2px solid transparent;
}

.nav a:hover { background: var(--bg); }

.nav a.active {
  background: var(--accent-soft);
  border-left-color: var(--accent);
  font-weight: 600;
}

.nav a .badge {
  float: right;
  font-size: 0.68rem;
  color: var(--accent);
  border: 1px solid currentColor;
  border-radius: 20px;
  padding: 0 6px;
  line-height: 1.5;
}

.nav a.done .badge {
  color: var(--ok);
}

main {
  padding: 48px 56px 120px;
  max-width: 980px;
}

/* --------------------------------------------------------------- content */

section { scroll-margin-top: 24px; padding-bottom: 20px; }

h1 {
  font-size: 2.1rem;
  letter-spacing: -0.02em;
  margin: 0 0 8px;
}

h2 {
  font-size: 1.45rem;
  letter-spacing: -0.015em;
  margin: 46px 0 4px;
  padding-top: 14px;
}

h3 {
  font-size: 1.05rem;
  margin: 26px 0 6px;
}

.eyebrow {
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent);
  font-weight: 700;
}

p { margin: 10px 0; }

ul { margin: 10px 0; padding-left: 20px; }

li { margin: 4px 0; }

code {
  font-family: var(--mono);
  font-size: 0.88em;
  background: #eceff3;
  padding: 1px 5px;
  border-radius: 4px;
}

pre {
  background: var(--code-bg);
  color: #e6e9ef;
  padding: 16px 18px;
  border-radius: var(--radius);
  overflow-x: auto;
  font-family: var(--mono);
  font-size: 0.85rem;
  line-height: 1.55;
}

pre code { background: none; padding: 0; color: inherit; font-size: inherit; }

.columns {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 18px;
  margin: 16px 0;
}

.card {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 16px 18px;
}

.card h4 { margin: 0 0 6px; font-size: 0.95rem; }

.card.bad { border-left: 3px solid var(--err); }
.card.good { border-left: 3px solid var(--ok); }

.note {
  background: var(--accent-soft);
  border-left: 3px solid var(--accent);
  padding: 12px 16px;
  border-radius: 0 var(--radius) var(--radius) 0;
  margin: 18px 0;
  font-size: 0.93rem;
}

/* -------------------------------------------------------------- exercise */

.exercise {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  margin: 26px 0;
  overflow: hidden;
}

.exercise-head {
  padding: 16px 20px;
  border-bottom: 1px solid var(--line);
  display: flex;
  align-items: center;
  gap: 12px;
}

.exercise-head .num {
  background: var(--accent);
  color: #fff;
  font-weight: 700;
  font-size: 0.8rem;
  width: 26px;
  height: 26px;
  border-radius: 50%;
  display: grid;
  place-items: center;
  flex: none;
}

.exercise-head h3 { margin: 0; font-size: 1.05rem; }

.exercise-body { padding: 16px 20px 0; font-size: 0.94rem; }

.exercise-body p:first-child { margin-top: 0; }

.editor-wrap { padding: 14px 20px 0; }

.CodeMirror {
  height: auto;
  border-radius: var(--radius);
  font-family: var(--mono);
  font-size: 0.85rem;
  line-height: 1.55;
  padding: 10px 4px;
}

.CodeMirror-scroll { min-height: 120px; }

.toolbar {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 14px 20px 18px;
  flex-wrap: wrap;
}

button {
  font-family: inherit;
  font-size: 0.88rem;
  font-weight: 600;
  border-radius: 7px;
  border: 1px solid var(--line);
  background: var(--panel);
  color: var(--ink);
  padding: 8px 14px;
  cursor: pointer;
}

button:hover { background: var(--bg); }

button.primary {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}

button.primary:hover { filter: brightness(1.07); }

button:disabled { opacity: 0.55; cursor: progress; }

.kbd-hint {
  color: var(--ink-soft);
  font-size: 0.78rem;
  margin-left: auto;
}

.result { padding: 0 20px 20px; display: none; }

.result.visible { display: block; }

.result-status {
  font-weight: 700;
  font-size: 0.9rem;
  padding: 10px 14px;
  border-radius: 8px;
  margin-bottom: 10px;
}

.result-status.pass { background: var(--ok-soft); color: var(--ok); }
.result-status.fail { background: var(--err-soft); color: var(--err); }

.test-list { list-style: none; padding: 0; margin: 0; font-size: 0.87rem; }

.test-list li {
  display: flex;
  gap: 9px;
  padding: 6px 2px;
  border-bottom: 1px solid var(--line);
  align-items: baseline;
}

.test-list li:last-child { border-bottom: none; }

.test-list .mark { font-weight: 700; flex: none; width: 16px; }
.test-list li.pass .mark { color: var(--ok); }
.test-list li.fail .mark { color: var(--err); }

.test-list .why {
  display: block;
  color: var(--err);
  font-family: var(--mono);
  font-size: 0.8rem;
  margin-top: 2px;
}

.console {
  background: var(--code-bg);
  color: #cdd3dc;
  font-family: var(--mono);
  font-size: 0.8rem;
  padding: 12px 14px;
  border-radius: 8px;
  margin-top: 12px;
  white-space: pre-wrap;
  overflow-x: auto;
}

.console-label {
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--ink-soft);
  font-weight: 700;
  margin-top: 14px;
}

.console.error { color: #ffb4a8; }

.runner-status {
  position: fixed;
  right: 18px;
  bottom: 16px;
  font-size: 0.75rem;
  color: var(--ink-soft);
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 20px;
  padding: 5px 12px;
}

.runner-status .dot {
  display: inline-block;
  width: 7px;
  height: 7px;
  border-radius: 50%;
  background: var(--ok);
  margin-right: 6px;
}

.runner-status.down .dot { background: var(--err); }

@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar { position: static; height: auto; }
  main { padding: 28px 20px 80px; }
  .columns { grid-template-columns: 1fr; }
}