:root {
  --bg: #f6f7fb;
  --card: #ffffff;
  --ink: #1c2333;
  --muted: #5c6478;
  --accent: #3b5bdb;
  --accent-ink: #ffffff;
  --chip-bg: #e9ecf5;
  --answer-bg: #f0f6f0;
  --answer-edge: #2f9e44;
  --edge: #dde1ec;
  --kbd-bg: #eef0f7;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #12151d;
    --card: #1b2030;
    --ink: #e8ebf4;
    --muted: #9aa3b8;
    --accent: #5c7cfa;
    --accent-ink: #ffffff;
    --chip-bg: #262c40;
    --answer-bg: #1a2a1e;
    --answer-edge: #51cf66;
    --edge: #2b3248;
    --kbd-bg: #262c40;
  }
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--ink);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 1.5rem 1rem 3rem;
}

header { text-align: center; margin-bottom: 1rem; }
header h1 { margin: 0 0 0.25rem; font-size: 1.9rem; }
.tagline { margin: 0; color: var(--muted); font-size: 0.95rem; }

nav#topics {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  justify-content: center;
  margin: 0.5rem 0 1.5rem;
}
.chip {
  border: 1px solid var(--edge);
  background: var(--chip-bg);
  color: var(--ink);
  border-radius: 999px;
  padding: 0.45rem 1rem;
  font-size: 0.9rem;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}
.chip:hover { border-color: var(--accent); }
.chip.active {
  background: var(--accent);
  border-color: var(--accent);
  color: var(--accent-ink);
}

main { width: 100%; max-width: 760px; }

.card {
  background: var(--card);
  border: 1px solid var(--edge);
  border-radius: 16px;
  padding: 1.5rem 1.75rem 1.75rem;
  box-shadow: 0 6px 24px rgba(10, 15, 40, 0.07);
}

.card-meta {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  margin-bottom: 0.75rem;
}
.badge {
  font-size: 0.75rem;
  font-weight: 600;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  padding: 0.25rem 0.6rem;
  border-radius: 6px;
  background: var(--accent);
  color: var(--accent-ink);
}
.badge.topic { background: var(--chip-bg); color: var(--ink); }
.count { margin-left: auto; color: var(--muted); font-size: 0.85rem; }

.instruction { color: var(--muted); margin: 0 0 0.25rem; }

.problem {
  font-size: 1.25rem;
  padding: 1rem 0.5rem 1.25rem;
  overflow-x: auto;
}

.check-area { margin-bottom: 1.1rem; }
.input-row { display: flex; gap: 0.6rem; }
#answer-input {
  flex: 1;
  min-width: 0;
  font-size: 1rem;
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  padding: 0.6rem 0.8rem;
  border: 1px solid var(--edge);
  border-radius: 10px;
  background: var(--bg);
  color: var(--ink);
}
#answer-input:focus { outline: 2px solid var(--accent); border-color: transparent; }
.syntax-hint { color: var(--muted); font-size: 0.8rem; margin: 0.5rem 0 0; }
.syntax-hint code {
  background: var(--kbd-bg);
  border-radius: 4px;
  padding: 0 0.3rem;
  font-size: 0.78rem;
}

.choice-row { display: flex; gap: 0.6rem; flex-wrap: wrap; }
.btn.choice { background: var(--chip-bg); color: var(--ink); border: 1px solid var(--edge); }
.btn.choice:hover { border-color: var(--accent); }
.btn.choice.right { background: var(--answer-edge); border-color: var(--answer-edge); color: #fff; }
.btn.choice.wrong { background: #c0392b; border-color: #c0392b; color: #fff; }

.feedback {
  margin-top: 0.7rem;
  padding: 0.55rem 0.9rem;
  border-radius: 8px;
  font-size: 0.95rem;
}
.feedback.good { background: var(--answer-bg); border: 1px solid var(--answer-edge); }
.feedback.bad { background: rgba(192, 57, 43, 0.12); border: 1px solid #c0392b; }
.feedback.warn { background: rgba(230, 150, 30, 0.12); border: 1px solid #d68910; }

nav#levels {
  display: flex;
  gap: 0.5rem;
  justify-content: center;
  margin: 1rem 0 0.5rem;
}
nav#levels .chip.level { font-weight: 600; }

.actions {
  display: flex;
  gap: 0.75rem;
  flex-wrap: wrap;
}
.btn {
  border: none;
  border-radius: 10px;
  padding: 0.65rem 1.2rem;
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
}
.btn.primary { background: var(--accent); color: var(--accent-ink); }
.btn.primary:hover { filter: brightness(1.1); }
.btn.secondary {
  background: var(--chip-bg);
  color: var(--ink);
  border: 1px solid var(--edge);
}
.btn.secondary:hover { border-color: var(--accent); }

kbd {
  background: var(--kbd-bg);
  border: 1px solid var(--edge);
  border-radius: 4px;
  padding: 0 0.35rem;
  font-size: 0.75rem;
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
}
.btn kbd { background: rgba(255, 255, 255, 0.18); border-color: transparent; color: inherit; }
.btn.secondary kbd { background: var(--kbd-bg); border-color: var(--edge); }

.answer {
  margin-top: 1.5rem;
  background: var(--answer-bg);
  border-left: 4px solid var(--answer-edge);
  border-radius: 10px;
  padding: 1rem 1.25rem;
  overflow-x: auto;
}
.answer h2 { font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.05em; color: var(--muted); margin: 0.75rem 0 0.4rem; }
.answer h2:first-child { margin-top: 0; }
.answer-line { font-size: 1.15rem; }
#steps { margin: 0; padding-left: 1.25rem; }
#steps li { margin: 0.4rem 0; line-height: 1.55; }

.hidden { display: none; }

footer { margin-top: 1.5rem; color: var(--muted); font-size: 0.85rem; }
footer p { margin: 0; }

@media (max-width: 520px) {
  .card { padding: 1.1rem 1rem 1.25rem; }
  .problem { font-size: 1.05rem; }
}