html, body { min-height: 100vh; }
body {
  background: #111;
  color: #eee;
  font-family: system-ui, -apple-system, Segoe UI, Roboto, sans-serif;
  margin: 0;
}

:root {
  --accent: #aaa;
  --accent-soft: rgba(170, 170, 170, 0.45);
  --muted-text: #b3b3b3;
}

/* ─── Slide-deck shell ───────────────────────────────────────────────── */
/* Each flow page fills the viewport so the app reads as a deck, not a
   scrolling article. Chapter nav pins to the top, page-nav falls to the
   bottom. Content flexes vertically between. */
.app-shell {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  max-width: 1040px;
  margin: 0 auto;
  padding: 1rem 1.25rem 1.4rem;
}
.app-main {
  flex: 1;
  display: flex;
  flex-direction: column;
  min-height: 0;
}
.slide-content {
  width: 100%;
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 0.2rem;
}
/* Pseudo-spacers flank the non-nav content so it vertically centers when
   the page is short, while tall pages flow naturally from the top. The
   spacer BEFORE page-nav (which has margin-top: auto) wins only if the
   content above leaves extra room — otherwise page-nav anchors the
   bottom of the viewport on tall screens. */
.slide-content::before {
  content: "";
  flex: 0.4 1 0;
  min-height: 0;
}
.slide-content > .page-nav { margin-top: auto; }

/* Bootstrap-default low-contrast greys are unreadable on a dark background.
   Push muted/secondary copy back into a legible range. */
.text-muted,
.text-body-secondary,
.lead.text-muted,
.fst-italic.text-muted {
  color: var(--muted-text) !important;
}

/* Replace Bootstrap's blue primary with a neutral grey across all buttons. */
.btn-primary {
  --bs-btn-bg: #555;
  --bs-btn-border-color: #6e6e6e;
  --bs-btn-color: #f0f0f0;
  --bs-btn-hover-bg: #666;
  --bs-btn-hover-border-color: #888;
  --bs-btn-hover-color: #fff;
  --bs-btn-active-bg: #444;
  --bs-btn-active-border-color: #888;
  --bs-btn-disabled-bg: #3a3a3a;
  --bs-btn-disabled-border-color: #555;
  --bs-btn-disabled-color: #999;
}

.btn-outline-primary {
  --bs-btn-color: #d0d0d0;
  --bs-btn-border-color: #555;
  --bs-btn-bg: transparent;
  --bs-btn-hover-bg: #2a2a2a;
  --bs-btn-hover-border-color: var(--accent);
  --bs-btn-hover-color: #fff;
  --bs-btn-active-bg: #3a3a3a;
  --bs-btn-active-border-color: var(--accent);
  --bs-btn-active-color: #fff;
  --bs-btn-disabled-color: #777;
  --bs-btn-disabled-border-color: #444;
}

.btn-outline-secondary {
  --bs-btn-color: #c0c0c0;
  --bs-btn-border-color: #555;
}

.container { max-width: 960px; }

.card.bg-dark { background: #1a1a1a !important; border-color: #2a2a2a; }
.card.bg-dark img { object-fit: cover; max-height: 220px; }

.lesson-body { line-height: 1.65; white-space: pre-wrap; }

.technical-term.faded { opacity: 0.35; font-style: italic; }

.scene-caption {
  max-width: 720px;
  margin: 0.4rem auto 0;
  text-align: center;
  line-height: 1.5;
  font-size: 0.8rem;
  opacity: 0.75;
}

/* Local fallback for Bootstrap `.text-center` so offline renders (atlas
   screenshots, any future air-gapped deploy) still center text without
   the CDN stylesheet. Bootstrap's `!important` form takes precedence
   when the CDN does load, so this doesn't fight anything. */
.text-center { text-align: center; }

/* Second-chance quiz state transitions: neutral → amber (first wrong) →
   red (locked). All options share a single transition so a click reads
   as a smooth state change rather than a flash. */
.quiz-option,
.image-quiz-option {
  transition: background-color 280ms ease, color 280ms ease,
              border-color 280ms ease, opacity 280ms ease,
              box-shadow 280ms ease;
}
.quiz-option { padding: 0.8rem 1rem; }
.quiz-option.correct,
.image-quiz-option.correct {
  background: #1a3a1a;
  color: #8f8;
  border-color: #4a4;
  box-shadow: 0 0 0 1px #4a4 inset;
}
.quiz-option.incorrect,
.image-quiz-option.incorrect {
  background: #3a1a1a;
  color: #f88;
  border-color: #a44;
  box-shadow: 0 0 0 1px #a44 inset;
  animation: shakeWrong 360ms ease-out 1;
}
/* First-wrong amber state: locally gray the picked option but leave the
   others interactive. The amber comes from the hint text underneath;
   the chosen option fades into a quiet disabled state. */
.quiz-option.disabled-wrong,
.image-quiz-option.disabled-wrong {
  background: #2a2118;
  color: #8a7a5a;
  border-color: #5a4a2a;
  opacity: 0.65;
}
@keyframes shakeWrong {
  0%, 100% { transform: translateX(0); }
  20%      { transform: translateX(-4px); }
  40%      { transform: translateX(4px); }
  60%      { transform: translateX(-3px); }
  80%      { transform: translateX(2px); }
}
@media (prefers-reduced-motion: reduce) {
  .quiz-option.incorrect,
  .image-quiz-option.incorrect { animation: none; }
}
/* Feedback text state colors (kept text-success/warning/danger classes
   from Bootstrap but with smoother transitions for the in-place swap). */
.quiz-feedback { transition: opacity 200ms ease; }
.quiz-feedback .text-warning { color: #f0b858 !important; }
.quiz-feedback .text-success { color: #7ddb7d !important; }
.quiz-feedback .text-danger  { color: #f27d7d !important; }

/* ─── Image-choice quiz (astrophotography) ───────────────────────────── */
.image-quiz-options {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: stretch;
  gap: 1rem;
}
.image-quiz-option {
  flex: 1 1 320px;
  max-width: 460px;
  padding: 0.6rem;
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
}
.image-quiz-option:disabled { opacity: 1; }
.image-quiz-img {
  width: 100%;
  aspect-ratio: 4 / 3;
  object-fit: cover;
  border-radius: 4px;
  display: block;
}
.image-quiz-label {
  font-size: 1rem;
  font-weight: 500;
  text-align: center;
}
.image-quiz-credit {
  font-size: 0.7rem;
  opacity: 0.5;
  text-align: center;
}
.image-quiz-option.correct { background: #1a3a1a; color: #8f8; border-color: #4a4; }
.image-quiz-option.incorrect { background: #3a1a1a; color: #f88; border-color: #a44; }
.image-quiz-option.disabled-wrong { background: #222; color: #888; border-color: #555; opacity: 0.6; }

.quiz-feedback { min-height: 2.5rem; font-size: 0.95rem; }

/* ─── Snap slider (iris + duration lessons) ──────────────────────────── */
.slider-viewer, .toggle-viewer {
  width: 100%;
  max-width: 820px;
  aspect-ratio: 16 / 9;
  margin: 0 auto;
  border-radius: 6px;
  overflow: hidden;
  background: #0a0a0a;
}
.slider-image, .toggle-image {
  width: 100%;
  height: 100%;
  object-fit: contain;
  display: block;
}
.slider-label, .toggle-label {
  font-size: 1.1rem;
  font-variant-numeric: tabular-nums;
  color: #ddd;
  margin-top: 0.4rem;
}
.slider-row input[type="range"] {
  flex: 1;
  max-width: 420px;
  accent-color: var(--accent);
  -webkit-appearance: none;
  appearance: none;
  height: 6px;
  background: #444;
  border-radius: 3px;
  outline: none;
}
.slider-row input[type="range"]::-webkit-slider-thumb {
  -webkit-appearance: none;
  width: 18px;
  height: 18px;
  border-radius: 50%;
  background: var(--accent);
  cursor: pointer;
}
.slider-row input[type="range"]::-moz-range-thumb {
  width: 18px;
  height: 18px;
  border-radius: 50%;
  background: var(--accent);
  border: none;
  cursor: pointer;
}

/* ─── Toggle group (sensitivity lesson) ──────────────────────────────── */
.toggle-group .toggle-btn {
  min-width: 6rem;
}
.toggle-group .toggle-btn.active {
  background: #444;
  border-color: var(--accent);
  color: #fff;
}

.compare-viewer {
  position: relative;
  width: 100%;
  max-width: 900px;
  aspect-ratio: 16 / 9;
  margin: 0 auto;
  cursor: ew-resize;
  user-select: none;
  touch-action: none;
  overflow: hidden;
  border-radius: 6px;
}
.compare-viewer img {
  position: absolute; top: 0; left: 0;
  width: 100%; height: 100%;
  object-fit: cover;
  display: block;
  pointer-events: none;
}
.compare-viewer .compare-overlay { clip-path: inset(0 0 0 50%); }
.compare-handle {
  position: absolute; top: 0; left: 50%;
  width: 3px; height: 100%;
  background: var(--accent);
  transform: translateX(-50%);
  pointer-events: none;
  box-shadow: 0 0 8px var(--accent-soft);
}
.compare-handle::before {
  content: '\25C0 \25B6';
  position: absolute; top: 50%; left: 50%;
  transform: translate(-50%, -50%);
  background: var(--accent); color: #000;
  font-weight: bold;
  padding: 0.3rem 0.7rem;
  border-radius: 16px;
  font-size: 0.72rem;
  letter-spacing: 0.2em;
}
.compare-label {
  position: absolute; bottom: 0.8rem;
  font-size: 0.85rem;
  padding: 0.3rem 0.6rem;
  background: rgba(0, 0, 0, 0.55);
  border-radius: 4px;
  pointer-events: none;
}
.compare-label-left { left: 0.8rem; }
.compare-label-right { right: 0.8rem; }

/* ─── Triangle summary (lesson 4) ────────────────────────────────────── */
.bucket-row {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: flex-start;
  gap: 1.5rem;
  margin: 1rem 0;
}
.bucket-item {
  flex: 1 1 180px;
  max-width: 220px;
  text-align: center;
}
.bucket-icon {
  font-size: 2.4rem;
  line-height: 1;
  margin-bottom: 0.6rem;
}
.bucket-label {
  line-height: 1.5;
  font-size: 0.95rem;
}
.bucket-label em {
  opacity: 0.85;
  font-size: 0.88rem;
}
.summary-body {
  font-size: 1.05rem;
  line-height: 1.7;
  opacity: 0.85;
  text-align: center;
  max-width: 700px;
  margin-left: auto;
  margin-right: auto;
}
.summary-insight {
  font-style: italic;
  font-size: 1.05rem;
  color: #ddd;
  text-align: center;
  max-width: 700px;
  margin-left: auto;
  margin-right: auto;
}

/* ─── Home (s0) ──────────────────────────────────────────────────────── */
.home-lead {
  font-size: 1.1rem;
  line-height: 1.6;
  max-width: 600px;
  margin: 0 auto;
}
.home-tagline {
  font-size: 0.95rem;
  color: #b3b3b3;
  line-height: 1.6;
  max-width: 600px;
  margin: 1rem auto 0;
}
.attribution {
  font-size: 0.8rem;
  color: #888;
  letter-spacing: 0.05em;
}

/* ─── Intro (s-drama) ────────────────────────────────────────────────── */
.intro-body {
  max-width: 820px;
  margin: 0 auto 1.5rem;
  line-height: 1.55;
  font-size: 1rem;
}
.intro-body p { margin-bottom: 0.6rem; }
.intro-divider {
  border-top: 1px solid #333;
  margin: 1.2rem auto 0.8rem;
  width: 600px;
  max-width: 80vw;
}
.intro-sidebar {
  max-width: 760px;
  margin: 0 auto 1rem;
  font-size: 0.9rem;
  color: #c5c5c5;
  line-height: 1.5;
}
.intro-sidebar-head {
  font-weight: 500;
  color: #ddd;
  margin-bottom: 0.4rem;
}

/* ─── Pre-interactive teaching showcase (s3, s6) ─────────────────────── */
.showcase-row {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: flex-end;
  gap: 1.5rem;
  margin: 0.5rem 0 1.2rem;
}
.showcase-card {
  flex: 1 1 220px;
  max-width: 340px;
  text-align: center;
}
.showcase-card img {
  width: 100%;
  height: auto;
  border-radius: 6px;
  display: block;
}
.showcase-card .showcase-label {
  font-size: 0.9rem;
  color: #ccc;
  margin-top: 0.4rem;
}

/* ─── Reference card polish ──────────────────────────────────────────── */
.ref-table {
  font-size: 0.85rem;
}
.ref-table th,
.ref-table td {
  color: #e0e0e0;
  vertical-align: middle;
}
.ref-table th {
  font-weight: 600;
  font-size: 0.82rem;
  letter-spacing: 0.02em;
  text-transform: uppercase;
  color: #cfcfcf;
}
.ref-table .small,
.ref-table .text-muted {
  color: #c0c0c0 !important;
  font-size: 0.78rem;
}
.ref-table .icon { font-size: 1rem; }
/* Override Bootstrap table-dark muted defaults for legibility. */
.table-dark {
  --bs-table-color: #e0e0e0;
  --bs-table-striped-color: #e0e0e0;
  --bs-table-bg: #1a1a1a;
  --bs-table-striped-bg: #202020;
  --bs-table-border-color: #2e2e2e;
}

/* ─── Page nav (Previous / Advance) ──────────────────────────────────── */
.page-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 1.5rem;
  gap: 1rem;
}
.page-nav .nav-form { margin: 0; }
.page-nav .nav-spacer { display: inline-block; }

/* ─── Chapter nav (top of every flow page) ───────────────────────────── */
/* Styled as a dim progress bar rather than a primary nav. Each chapter is
   a thin segment with a bottom border that fills in as the user
   progresses. Still clickable for back-navigation. */
.chapter-nav {
  margin-bottom: 1.25rem;
  flex-shrink: 0;
}
.chapter-list {
  display: flex;
  list-style: none;
  padding: 0;
  margin: 0;
  gap: 4px;
  font-size: 0.7rem;
}
.chapter-item {
  flex: 1 1 auto;
  min-width: 0;
}
.chapter-item a {
  display: block;
  padding: 0.35rem 0.4rem 0.45rem;
  color: #555;
  text-decoration: none;
  text-align: center;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  border-bottom: 2px solid #242424;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
  position: relative;
  transition: color 180ms ease;
}
/* Progress-bar fill-in: the underline is a zero-width bar that sweeps
   left→right when .chapter-nav is marked .progressed (added via JS on
   mount). The per-item --chapter-delay from the template staggers the
   sweep so passed chapters "fill in" one after another. */
.chapter-item a::after {
  content: "";
  position: absolute;
  left: 0;
  right: 100%;
  bottom: -2px;
  height: 2px;
  background: transparent;
  transition: right 520ms ease-out;
  transition-delay: var(--chapter-delay, 0ms);
}
.chapter-nav.progressed .chapter-item.passed a::after,
.chapter-nav.progressed .chapter-item.active a::after {
  right: 0;
}
.chapter-item.passed a::after { background: #5e5e5e; }
.chapter-item.active a::after { background: var(--accent); }
.chapter-item.passed a { color: #8a8a8a; }
.chapter-item.active a {
  color: #f0f0f0;
  font-weight: 500;
}
.chapter-item a:hover { color: #ddd; }
.chapter-item.upcoming a:hover::after {
  right: 70%;
  background: #3a3a3a;
}
@media (prefers-reduced-motion: reduce) {
  .chapter-item a::after { transition: none; transition-delay: 0s; }
}
.chapter-nav.progressed-instant .chapter-item a::after {
  transition: none;
  transition-delay: 0s;
}

/* ─── Paragraph reveal (Brilliant-style cascade on page enter) ───────── */
/* Direct children of .slide-content start hidden, fade in on a staggered
   delay set via --reveal-delay. Any user input (pointerdown / keydown /
   scroll) flips the shell to .reveal-skip, which drops the delay to 0
   and makes the remaining blocks snap in. */
.slide-content > * {
  opacity: 0;
  transform: translateY(6px);
  transition: opacity 420ms ease-out, transform 420ms ease-out;
  transition-delay: var(--reveal-delay, 0ms);
  will-change: opacity, transform;
}
.slide-content.reveal-ready > * {
  opacity: 1;
  transform: none;
}
.slide-content.reveal-skip > * {
  transition: none !important;
  opacity: 1;
  transform: none;
}
@media (prefers-reduced-motion: reduce) {
  .slide-content > * {
    opacity: 1;
    transform: none;
    transition: none;
  }
}

/* ─── Idle-glow nudge (60s dwell on Advance) ─────────────────────────── */
@keyframes glowPulse {
  0%, 100% { box-shadow: 0 0 0 0 rgba(200, 200, 200, 0.0); }
  50%      { box-shadow: 0 0 12px 3px rgba(200, 200, 200, 0.35); }
}
.btn.glow-nudge { animation: glowPulse 2.2s ease-in-out infinite; }
@media (prefers-reduced-motion: reduce) {
  .btn.glow-nudge { animation: none; outline: 1px solid var(--accent); }
}

/* ─── Auto-drift cue (widgets softly cycle while idle) ───────────────── */
/* The drift is driven by JS; this class provides a subtle shimmer on the
   range-track so users read the widget as "alive / touchable". */
.slider-row.auto-drifting input[type="range"],
.compare-viewer.auto-drifting .compare-handle {
  transition: filter 600ms ease;
  filter: brightness(1.05);
}