:root {
  --lesson-ink: #162238;
  --lesson-muted: #5f6f84;
  --lesson-blue: #16709a;
  --lesson-blue-dark: #0b4d70;
  --lesson-orange: #eb6119;
  --lesson-green: #16834a;
  --lesson-red: #c43f3f;
  --lesson-purple: #7654b5;
  --lesson-line: #d9e3ea;
  --lesson-soft: #eff6f9;
  --lesson-paper: #fff;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  color: var(--lesson-ink);
  background:
    radial-gradient(circle at 90% 4%, rgba(235, 97, 25, .1), transparent 22rem),
    #f3f7f9;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Noto Sans JP",
    "Yu Gothic UI", "Yu Gothic", Meiryo, sans-serif;
  line-height: 1.7;
}

button, input, select { font: inherit; }
button, input, select, textarea { accent-color: var(--lesson-blue); }
:focus-visible { outline: 3px solid var(--lesson-orange); outline-offset: 3px; }

.lesson-page {
  width: min(1040px, calc(100% - 32px));
  margin: 0 auto;
  padding: 34px 0 64px;
}

.lesson-hero {
  position: relative;
  overflow: hidden;
  margin-bottom: 22px;
  padding: clamp(24px, 5vw, 42px);
  color: #fff;
  background: linear-gradient(120deg, var(--lesson-blue-dark), var(--lesson-blue));
  border-left: 7px solid var(--lesson-orange);
  border-radius: 18px;
  box-shadow: 0 15px 35px rgba(13, 71, 100, .16);
}

.lesson-hero::after {
  content: "";
  position: absolute;
  right: -60px;
  bottom: -100px;
  width: 220px;
  height: 220px;
  border: 35px solid rgba(255, 255, 255, .08);
  border-radius: 50%;
}

.lesson-kicker {
  margin: 0 0 8px;
  color: #ffd6c2;
  font-size: .78rem;
  font-weight: 800;
  letter-spacing: .12em;
}

.lesson-hero h1 {
  max-width: 800px;
  margin: 0;
  font-size: clamp(1.8rem, 5vw, 3rem);
  line-height: 1.2;
  letter-spacing: -.025em;
}

.lesson-hero p:last-child {
  max-width: 760px;
  margin: 12px 0 0;
  color: rgba(255, 255, 255, .84);
}

.objective-list {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 10px;
  margin: 0 0 22px;
  padding: 0;
  list-style: none;
}

.objective-list li {
  padding: 13px 14px;
  border: 1px solid var(--lesson-line);
  border-radius: 12px;
  background: rgba(255, 255, 255, .82);
  color: var(--lesson-muted);
  font-size: .84rem;
}

.objective-list li::before {
  content: "✓";
  margin-right: 7px;
  color: var(--lesson-green);
  font-weight: 900;
}

.lesson-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
}

.lesson-grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.lesson-grid + .lesson-panel, .lesson-panel + .lesson-panel { margin-top: 18px; }

.lesson-panel {
  min-width: 0;
  padding: clamp(18px, 3vw, 26px);
  border: 1px solid var(--lesson-line);
  border-radius: 16px;
  background: var(--lesson-paper);
  box-shadow: 0 6px 18px rgba(27, 61, 82, .07);
}

.lesson-panel h2 {
  margin: 0 0 14px;
  color: var(--lesson-ink);
  font-size: 1.25rem;
  line-height: 1.35;
}

.lesson-panel h3 { margin: 18px 0 8px; font-size: 1rem; }
.lesson-panel p { margin: 0 0 12px; }
.lesson-panel p:last-child { margin-bottom: 0; }

.control-row {
  display: grid;
  grid-template-columns: minmax(120px, auto) 1fr auto;
  align-items: center;
  gap: 12px;
  margin: 12px 0;
}

.control-row label { color: var(--lesson-muted); font-weight: 700; }
.control-row input[type="range"] { width: 100%; }

.lesson-button {
  min-height: 44px;
  padding: 10px 17px;
  border: 0;
  border-radius: 10px;
  color: #fff;
  background: var(--lesson-blue);
  cursor: pointer;
  font-weight: 800;
}

.lesson-button:hover { background: var(--lesson-blue-dark); }
.lesson-button.secondary { background: #66758a; }
.lesson-button.orange { background: var(--lesson-orange); }
.lesson-button:disabled { opacity: .55; cursor: not-allowed; }

.button-row { display: flex; flex-wrap: wrap; gap: 9px; }

.choice-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 10px;
}

.choice-button {
  min-height: 48px;
  padding: 10px 12px;
  border: 1px solid var(--lesson-line);
  border-radius: 10px;
  color: var(--lesson-ink);
  background: #fff;
  cursor: pointer;
  font-weight: 700;
  text-align: left;
}

.choice-button:hover { border-color: var(--lesson-blue); background: #f2f9fc; }
.choice-button.correct { border-color: var(--lesson-green); background: #e8f7ee; }
.choice-button.incorrect { border-color: var(--lesson-red); background: #fff0f0; }

.feedback {
  display: none;
  margin-top: 14px;
  padding: 13px 15px;
  border-left: 5px solid var(--lesson-blue);
  border-radius: 9px;
  background: var(--lesson-soft);
}

.feedback.show { display: block; }
.feedback.correct { border-color: var(--lesson-green); background: #eaf8ef; }
.feedback.incorrect { border-color: var(--lesson-red); background: #fff0f0; }

.metric-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

.metric {
  padding: 14px;
  border: 1px solid var(--lesson-line);
  border-radius: 12px;
  background: #f8fbfc;
  text-align: center;
}

.metric strong {
  display: block;
  color: var(--lesson-blue);
  font-size: 1.4rem;
}

.flow {
  display: flex;
  align-items: stretch;
  gap: 8px;
  overflow-x: auto;
  padding-bottom: 5px;
}

.flow-step {
  min-width: 130px;
  flex: 1;
  padding: 12px;
  border: 1px solid var(--lesson-line);
  border-radius: 10px;
  background: #fff;
  text-align: center;
  font-size: .82rem;
  font-weight: 700;
}

.flow-arrow { align-self: center; color: var(--lesson-orange); font-weight: 900; }

.viz {
  position: relative;
  min-height: 230px;
  overflow: hidden;
  border: 1px solid var(--lesson-line);
  border-radius: 12px;
  background:
    linear-gradient(rgba(32, 112, 154, .06) 1px, transparent 1px),
    linear-gradient(90deg, rgba(32, 112, 154, .06) 1px, transparent 1px),
    #fbfdfe;
  background-size: 24px 24px;
}

.bar-row {
  display: grid;
  grid-template-columns: 110px 1fr 48px;
  align-items: center;
  gap: 9px;
  margin: 9px 0;
  font-size: .82rem;
}

.bar-track {
  height: 15px;
  overflow: hidden;
  border-radius: 99px;
  background: #e3ebf0;
}

.bar-fill {
  height: 100%;
  border-radius: inherit;
  background: var(--lesson-blue);
  transition: width .3s ease;
}

.tag {
  display: inline-flex;
  padding: 3px 8px;
  border-radius: 999px;
  color: #0b5d82;
  background: #e8f5fa;
  font-size: .72rem;
  font-weight: 800;
}

.formula {
  padding: 13px;
  overflow-x: auto;
  border: 1px solid var(--lesson-line);
  border-radius: 9px;
  background: #f7fafc;
  font-family: "SFMono-Regular", Consolas, monospace;
  text-align: center;
}

.table-wrap { overflow-x: auto; }
.lesson-table { width: 100%; border-collapse: collapse; font-size: .84rem; }
.lesson-table th, .lesson-table td {
  padding: 9px 10px;
  border: 1px solid var(--lesson-line);
  text-align: left;
}
.lesson-table th { background: var(--lesson-soft); }

.status {
  min-height: 28px;
  margin-top: 12px;
  color: var(--lesson-muted);
  font-weight: 700;
}

@media (max-width: 760px) {
  .lesson-page { width: min(100% - 20px, 1040px); padding-top: 20px; }
  .objective-list, .lesson-grid, .lesson-grid.three { grid-template-columns: 1fr; }
  .choice-grid { grid-template-columns: 1fr; }
  .metric-grid { grid-template-columns: repeat(2, 1fr); }
  .control-row { grid-template-columns: 1fr auto; }
  .control-row input { grid-column: 1 / -1; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: .01ms !important; }
}