:root {
  --ink: #1b2a24;
  --muted: #5b6b64;
  --line: #d8e0dc;
  --green: #1f7a4d;
  --green-dark: #17603c;
  --paper: #f6f8f7;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  background: var(--paper);
  color: var(--ink);
  font: 16px/1.5 "Segoe UI", system-ui, sans-serif;
}

main { max-width: 720px; margin: 0 auto; padding: 32px 20px 64px; }

header h1 { margin: 0; color: var(--green-dark); font-size: 1.8em; }
header p { margin: 4px 0 18px; color: var(--muted); }

textarea, input[type="text"], select {
  display: block;
  width: 100%;
  padding: 9px 12px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
  color: var(--ink);
  font: inherit;
}
textarea { margin-bottom: 10px; resize: vertical; }

button {
  padding: 7px 14px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
  color: var(--ink);
  font: inherit;
  cursor: pointer;
}
button:hover { border-color: var(--green); }
button:focus-visible, textarea:focus-visible, input:focus-visible, select:focus-visible { outline: 2px solid var(--green); outline-offset: 1px; }
button.primary { border-color: var(--green); background: var(--green); color: #fff; }
button.primary:hover { background: var(--green-dark); }

.tabs, .depth { display: flex; gap: 4px; }
.tabs { margin-bottom: 16px; }
.depth { margin: 14px 0 4px; }
.tabs .on, .depth .on { border-color: var(--green); background: #e4f1ea; color: var(--green-dark); }

#build-form { display: flex; flex-wrap: wrap; gap: 12px; margin-top: 12px; align-items: flex-end; }
.field { flex: 1 1 100%; }
.field.short { flex: 1 1 180px; }
.field label { display: block; margin-bottom: 3px; font-size: .92em; font-weight: 600; }
.check { flex: 1 1 100%; display: flex; gap: 8px; align-items: center; }
#build-form > button { flex: 0 0 auto; }

.problem {
  margin-top: 16px;
  padding: 10px 12px;
  border-left: 4px solid #b3402e;
  border-radius: 4px;
  background: #fbeeeb;
  color: #7c2a1e;
}

.answer {
  margin-top: 18px;
  padding: 18px;
  border: 1px solid var(--line);
  border-radius: 10px;
  background: #fff;
}
.answer h2 { margin: 0 0 12px; font-size: 1.25em; }

h3 { margin: 16px 0 4px; font-size: 1em; }
ul, ol { margin: 0; padding-left: 20px; }
section p { margin: 0; }

.formula code {
  display: block;
  padding: 10px 12px;
  border-radius: 8px;
  background: #eef3f0;
  font: 1em Consolas, "Courier New", monospace;
  overflow-wrap: anywhere;
}
.buttons { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 8px; }

.message { min-height: 1.5em; margin: 6px 0 0; color: var(--green-dark); font-size: .92em; }
.note { margin: 16px 0 0; color: var(--muted); font-size: .9em; }

.others, .browse { margin-top: 18px; }
.browse summary { color: var(--green-dark); cursor: pointer; font-weight: 600; }

button.link {
  display: block;
  padding: 3px 0;
  border: 0;
  background: none;
  color: var(--green-dark);
  text-align: left;
  text-decoration: underline;
}

a { color: var(--green-dark); }
.tabs { flex-wrap: wrap; }
.tab {
  padding: 7px 14px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
  color: var(--ink);
  text-decoration: none;
}
.tab:hover { border-color: var(--green); }
.tab:focus-visible { outline: 2px solid var(--green); outline-offset: 1px; }
.tab.on { border-color: var(--green); background: #e4f1ea; color: var(--green-dark); }

.section-title { margin: 26px 0 6px; font-size: 1.15em; }
.lesson-row { margin: 8px 0 12px; }
.lesson-row p { margin: 0; color: var(--muted); font-size: .94em; }
.level { margin: 0 0 4px; color: var(--muted); font-size: .9em; }
.next { margin-top: 20px; }

code.example, .page code {
  display: block;
  margin: 6px 0 2px;
  padding: 8px 12px;
  border-radius: 8px;
  background: #eef3f0;
  font: 1em Consolas, "Courier New", monospace;
  overflow-wrap: anywhere;
}
.answer > p { margin: 4px 0; }

.grid-wrap { max-width: 100%; margin: 10px 0; overflow-x: auto; }
.grid { border-collapse: collapse; font-size: .92em; }
.grid th, .grid td { min-width: 44px; padding: 3px 8px; border: 1px solid var(--line); white-space: pre; }
.grid th { background: #eef3f0; color: var(--muted); font-weight: 500; text-align: center; }
.grid td { background: #fff; }
.grid td.target { background: #fdf3cf; font-weight: 700; text-align: center; }

.practice-input { margin-bottom: 8px; font-family: Consolas, "Courier New", monospace !important; }
.feedback { min-height: 1.5em; margin: 8px 0 0; font-weight: 600; }
.feedback.right { color: var(--green-dark); }
.feedback.wrong { color: #a33a29; }
.feedback.unknown { color: #7a5b00; }

footer { margin-top: 40px; padding-top: 12px; border-top: 1px solid var(--line); color: var(--muted); font-size: .9em; }
.page nav.top { margin-bottom: 14px; }