:root {
  color-scheme: light dark;
  --font-sans: system-ui, -apple-system, "Segoe UI", "Hiragino Kaku Gothic ProN",
    "Noto Sans JP", sans-serif;
  --color-link: #1d4ed8;
  --color-accent-contrast: #ffffff;
  --color-border: #e5e7eb;
  --color-muted: #4b5563;
  --color-surface: #f9fafb;
  --radius: 10px;
  --space-2: 1rem;
  --space-3: 1.5rem;
  --space-4: 2.5rem;
  --content-width: 960px;
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-link: #93c5fd;
    --color-accent-contrast: #0b0f19;
    --color-border: #333a45;
    --color-muted: #b6bec9;
    --color-surface: #131824;
  }
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: var(--font-sans);
  font-size: 1rem;
  line-height: 1.7;
}

h1,
h2,
h3 {
  line-height: 1.3;
}

a {
  color: var(--color-link);
}

/* keyboard-visible focus ring for all interactive elements */
a:focus-visible,
summary:focus-visible {
  outline: 3px solid var(--color-link);
  outline-offset: 2px;
  border-radius: 4px;
}

.skip-link {
  position: absolute;
  left: -999px;
  top: 0;
  z-index: 100;
  background: var(--color-link);
  color: var(--color-accent-contrast);
  padding: 0.75rem 1.25rem;
  border-radius: 0 0 8px 0;
  font-weight: 600;
}

.skip-link:focus {
  left: 0;
}

.site-header {
  border-bottom: 1px solid var(--color-border);
  padding: var(--space-3) 1.5rem var(--space-2);
}

.site-header__row {
  max-width: var(--content-width);
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.site-header__brand {
  margin: 0;
  font-weight: 700;
  font-size: 1.05rem;
}

.site-header__tagline {
  max-width: var(--content-width);
  margin: 0 auto;
  color: var(--color-muted);
}

.lang-switch {
  display: flex;
  gap: 0.5rem;
}

.lang-switch a {
  padding: 0.4rem 0.9rem;
  border: 1px solid var(--color-border);
  border-radius: 999px;
  color: inherit;
  text-decoration: none;
  font-size: 0.875rem;
}

.lang-switch a[aria-current="page"] {
  background: var(--color-link);
  border-color: var(--color-link);
  color: var(--color-accent-contrast);
  font-weight: 600;
}

.lang-switch a:hover,
.lang-switch a:focus-visible {
  border-color: var(--color-link);
}

main {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: var(--space-4) 1.5rem;
  display: grid;
  gap: var(--space-4);
}

main h2 {
  font-size: 1.35rem;
  margin: 0 0 0.75rem;
}

.topic-tree {
  display: grid;
  gap: 0.75rem;
}

.topic-tree details {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 0.9rem 1.1rem;
}

.topic-tree summary {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  cursor: pointer;
  font-weight: 600;
  font-size: 1.05rem;
  list-style: none;
}

.topic-tree summary::-webkit-details-marker {
  display: none;
}

.topic-tree summary::before {
  content: "";
  width: 0.5rem;
  height: 0.5rem;
  flex: none;
  border-right: 2px solid currentColor;
  border-bottom: 2px solid currentColor;
  transform: rotate(-45deg);
  transition: transform 0.15s ease;
}

.topic-tree details[open] summary::before {
  transform: rotate(45deg);
}

.topic-tree__intro {
  margin: 0.5rem 0 0;
  color: var(--color-muted);
  font-size: 0.9rem;
}

@media (prefers-reduced-motion: reduce) {
  .topic-tree summary::before {
    transition: none;
  }
}

.topic-tree ul {
  list-style: none;
  margin: 0.75rem 0 0;
  padding: 0 0 0 1.15rem;
  border-left: 2px solid var(--color-border);
  display: grid;
  gap: 0.4rem;
}

.topic-tree li a {
  display: inline-block;
  padding: 0.2rem 0;
  text-decoration: none;
}

.topic-tree li a:hover,
.topic-tree li a:focus-visible {
  text-decoration: underline;
}

.site-footer {
  border-top: 1px solid var(--color-border);
  margin-top: var(--space-4);
  padding: var(--space-3) 1.5rem;
  text-align: center;
}

.site-footer p {
  max-width: var(--content-width);
  margin: 0 auto;
  color: var(--color-muted);
  font-size: 0.85rem;
}

/* ---- topic page: breadcrumb ---- */

.breadcrumb ol {
  list-style: none;
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
  margin: 0;
  padding: 0;
  font-size: 0.85rem;
  color: var(--color-muted);
}

.breadcrumb li:not(:last-child)::after {
  content: "›";
  margin-left: 0.35rem;
}

.breadcrumb a {
  color: inherit;
  text-decoration: underline;
}

.breadcrumb li[aria-current="page"] {
  color: inherit;
  font-weight: 600;
}

/* ---- topic page: slideshow ---- */

.slideshow__meta {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: var(--space-2);
  flex-wrap: wrap;
  margin: var(--space-2) 0 var(--space-3);
}

.slideshow__meta h1 {
  margin: 0;
  font-size: clamp(1.4rem, 1.1rem + 1vw, 1.9rem);
}

.slideshow__progress {
  color: var(--color-muted);
  font-size: 0.9rem;
  white-space: nowrap;
}

.slide {
  outline: none;
}

/* When JS is disabled, .slide elements are never hidden, so all slides
   render as one continuous article; the divider keeps that readable. */
.slide + .slide {
  border-top: 1px solid var(--color-border);
  margin-top: var(--space-3);
  padding-top: var(--space-3);
}

.slide h2 {
  margin-top: 0;
}

.slide .related-links {
  list-style: none;
  padding: 0;
  margin: 0.75rem 0 0;
  display: grid;
  gap: 0.4rem;
}

.slideshow__controls {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-2);
  margin-top: var(--space-3);
}

.slideshow__controls button {
  font: inherit;
  font-weight: 600;
  cursor: pointer;
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  color: inherit;
  border-radius: 999px;
  padding: 0.55rem 1.1rem;
  min-height: 44px;
}

.slideshow__controls button:disabled {
  opacity: 0.4;
  cursor: not-allowed;
}

.slideshow__controls button:hover:not(:disabled),
.slideshow__controls button:focus-visible {
  border-color: var(--color-link);
}

.slideshow__dots {
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

.slideshow__dots button {
  width: 0.6rem;
  height: 0.6rem;
  min-height: 0;
  padding: 0;
  border: none;
  border-radius: 50%;
  background: var(--color-border);
}

.slideshow__dots button[aria-current="true"] {
  background: var(--color-link);
}

/* Controls only make sense once JS has taken over pagination */
.slideshow:not(.is-enhanced) .slideshow__controls,
.slideshow:not(.is-enhanced) .slideshow__progress {
  display: none;
}