:root {
  color-scheme: light dark;
  --bg: #f4f7fb;
  --bg-mesh: radial-gradient(1200px 600px at 100% -10%, rgba(29, 78, 216, 0.07), transparent 55%),
    radial-gradient(900px 500px at -5% 20%, rgba(13, 148, 136, 0.06), transparent 50%),
    #f4f7fb;
  --panel: #ffffff;
  --border: #dbe4ee;
  --text: #0f172a;
  --muted: #475569;
  --accent: #1d4ed8;
  --accent-soft: #dbeafe;
  /* Accents for cards / highlights */
  --c-blue: #2563eb;
  --c-teal: #0d9488;
  --c-violet: #7c3aed;
  --c-amber: #d97706;
  --c-rose: #e11d48;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
  background: var(--bg-mesh);
  color: var(--text);
  line-height: 1.65;
}
header {
  background: linear-gradient(125deg, #0f172a 0%, #1e3a8a 38%, #1d4ed8 62%, #0e7490 100%);
  color: white;
  padding: 32px 24px;
  box-shadow: 0 12px 40px rgba(15, 23, 42, 0.18);
}
header h1 {
  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.2);
}
header p {
  opacity: 0.95;
  max-width: 70ch;
}
main {
  max-width: 1200px;
  margin: 0 auto;
  padding: 24px;
}
.grid {
  display: grid;
  grid-template-columns: 280px 1fr;
  gap: 24px;
  align-items: start;
}
nav, section, .card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 20px;
  box-shadow: 0 2px 12px rgba(15, 23, 42, 0.04);
}
nav {
  position: sticky;
  top: 24px;
  border-left: 4px solid var(--c-teal);
  background: linear-gradient(180deg, #ffffff 0%, #f8fafc 100%);
}
nav ul {
  list-style: none;
  padding: 0;
  margin: 0;
}
nav li { margin: 10px 0; }
nav ul.nav-sub {
  margin: 6px 0 0;
  padding-left: 14px;
  border-left: 2px solid var(--border);
}
nav ul.nav-sub li {
  margin: 6px 0;
  font-size: 0.92rem;
}
nav a {
  color: var(--accent);
  text-decoration: none;
}
nav a:hover { text-decoration: underline; color: #1e40af; }
nav a[aria-current="page"] {
  display: inline-block;
  font-weight: 700;
  color: var(--text);
  padding: 6px 10px;
  border-radius: 10px;
  background: linear-gradient(90deg, rgba(37, 99, 235, 0.14), rgba(13, 148, 136, 0.12));
  border: 1px solid rgba(37, 99, 235, 0.18);
}
section > h2 {
  letter-spacing: 0.1px;
  padding-bottom: 8px;
  border-bottom: 2px solid transparent;
  border-image: linear-gradient(90deg, var(--c-blue), var(--c-teal)) 1;
  border-image-slice: 1;
}
section > h3 {
  letter-spacing: 0.1px;
  color: #1e3a8a;
}
section p {
  max-width: 85ch;
}
section blockquote {
  margin: 12px 0;
  padding: 8px 12px;
  border-left: 4px solid var(--c-teal);
  border-radius: 0 8px 8px 0;
  background: linear-gradient(90deg, rgba(13, 148, 136, 0.08), #f8fafc);
}
section blockquote p {
  margin: 0;
  font-size: 0.9rem;
  color: var(--muted);
  line-height: 1.5;
}
small, .muted { color: var(--muted); }
.footnotes {
  margin-top: 20px;
  padding-top: 14px;
  border-top: 1px solid var(--border);
  font-size: 0.92rem;
  color: var(--muted);
}
.footnotes p {
  margin: 0;
  max-width: 85ch;
}
.footnotes a.fn-back,
a.fn-ref {
  text-decoration: none;
  font-weight: 700;
  color: var(--accent);
}
.footnotes a.fn-back:hover,
a.fn-ref:hover {
  text-decoration: underline;
}
.table {
  width: 100%;
  border-collapse: collapse;
}
.table th, .table td {
  border: 1px solid var(--border);
  padding: 10px;
  text-align: left;
  vertical-align: top;
}
.table th {
  background: linear-gradient(180deg, #eef2ff 0%, #e0e7ff 100%);
  color: #1e3a8a;
}
.table tr:hover td { background: #f0f9ff; }
.footer {
  padding: 24px;
  text-align: center;
  color: var(--muted);
  border-top: 1px solid var(--border);
  background: linear-gradient(180deg, transparent, rgba(29, 78, 216, 0.04));
}
@media (max-width: 900px) {
  .grid { grid-template-columns: 1fr; }
  nav { position: static; }
}

/* Illustration / diagram placeholders */
.placeholder-media {
  border: 2px dashed rgba(13, 148, 136, 0.35);
  border-radius: 12px;
  background: linear-gradient(145deg, #f0fdfa 0%, #f1f5f9 100%);
  min-height: 140px;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;
  margin: 16px 0;
  color: var(--muted);
  font-size: 0.95rem;
  text-align: center;
}
.placeholder-media strong {
  display: block;
  color: var(--text);
  margin-bottom: 6px;
}
.drafting-banner {
  border: 3px dashed rgba(29, 78, 216, 0.35);
  border-radius: 16px;
  background: linear-gradient(160deg, #eff6ff 0%, #f1f5f9 45%, #e2e8f0 100%);
  min-height: min(58vh, 480px);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 16px;
  padding: 48px 32px;
  margin: 8px 0 24px;
  text-align: center;
  box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.6);
}
.drafting-banner-label {
  font-size: clamp(2rem, 6vw, 3.25rem);
  font-weight: 800;
  letter-spacing: 0.14em;
  color: #1e3a8a;
  line-height: 1.1;
}
.drafting-banner-note {
  max-width: 42ch;
  font-size: 1rem;
  color: var(--muted);
  line-height: 1.5;
}
.mermaid {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 12px;
  margin: 16px 0;
  overflow-x: auto;
}
.power-cycle-wrap {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 20px;
  margin: 16px 0;
}
.power-cycle-wrap img,
.power-cycle-wrap svg {
  display: block;
  max-width: 100%;
  height: auto;
  margin: 0 auto;
}
.doc-note {
  font-size: 0.9rem;
  color: var(--muted);
  border-left: 4px solid var(--c-teal);
  padding: 8px 12px;
  margin: 12px 0;
  background: linear-gradient(90deg, rgba(13, 148, 136, 0.08), #f8fafc);
  border-radius: 0 8px 8px 0;
}

/* Collapsible "accordion" sections */
.accordion-item {
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--panel);
  margin: 12px 0;
  overflow: hidden;
}
.accordion-item summary {
  cursor: pointer;
  padding: 12px 14px;
  background: linear-gradient(90deg, var(--accent-soft) 0%, #eff6ff 100%);
  color: var(--text);
  list-style: none;
}
.accordion-item summary::-webkit-details-marker { display: none; }
.accordion-item summary::before {
  content: "▸";
  display: inline-block;
  margin-right: 8px;
  color: var(--c-blue);
  transition: transform 0.2s ease;
  font-size: 0.85em;
}
.accordion-item[open] summary::before {
  transform: rotate(90deg);
}
.accordion-item:nth-child(3n+1) summary {
  background: linear-gradient(90deg, #dbeafe 0%, #eff6ff 100%);
}
.accordion-item:nth-child(3n+2) summary {
  background: linear-gradient(90deg, #ccfbf1 0%, #f0fdfa 100%);
}
.accordion-item:nth-child(3n) summary {
  background: linear-gradient(90deg, #ede9fe 0%, #f5f3ff 100%);
}
.accordion-item p {
  margin: 10px 14px 12px;
}
.accordion-item[open] summary {
  border-bottom: 1px solid var(--border);
}

/* Optional card blocks for future sections */
.cards {
  display: grid;
  gap: 12px;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
  margin: 14px 0;
}
.card-link {
  display: block;
  text-decoration: none;
  color: inherit;
  border: 1px solid var(--border);
  border-left: 4px solid var(--card-accent, var(--c-blue));
  border-radius: 12px;
  background: linear-gradient(165deg, #ffffff 0%, #f8fafc 100%);
  padding: 14px;
  transition: transform 0.12s ease, box-shadow 0.12s ease, border-color 0.12s ease;
}
.card-link:nth-child(1) { --card-accent: var(--c-blue); }
.card-link:nth-child(2) { --card-accent: var(--c-teal); }
.card-link:nth-child(3) { --card-accent: var(--c-violet); }
.card-link:nth-child(4) { --card-accent: var(--c-amber); }
.card-link:nth-child(5) { --card-accent: var(--c-rose); }
.card-link:nth-child(6) { --card-accent: #0f766e; }
.card-link:hover {
  transform: translateY(-2px);
  border-color: var(--card-accent);
  box-shadow: 0 10px 28px rgba(37, 99, 235, 0.12);
}
.card-link strong {
  display: block;
  margin-bottom: 6px;
  color: #1e3a8a;
}

/* Home / Start here: enforce square cards without visual holes */
.start-here-grid {
  grid-template-columns: repeat(2, minmax(0, 1fr));
}
.start-here-grid .card-link {
  min-height: 0;
  display: grid;
  align-content: start;
  gap: 8px;
}
.start-here-grid .card-link .muted {
  display: block;
}

@media (max-width: 700px) {
  .start-here-grid {
    grid-template-columns: 1fr;
  }
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0b1220;
    --bg-mesh: radial-gradient(1000px 500px at 100% 0%, rgba(59, 130, 246, 0.12), transparent 55%),
      radial-gradient(800px 400px at 0% 30%, rgba(20, 184, 166, 0.08), transparent 45%),
      #0b1220;
    --panel: #111c2f;
    --border: #2a3f5c;
    --text: #e6edf7;
    --muted: #9fb0c7;
    --accent: #93c5fd;
    --accent-soft: #1a2744;
    --c-blue: #60a5fa;
    --c-teal: #2dd4bf;
    --c-violet: #a78bfa;
    --c-amber: #fbbf24;
    --c-rose: #fb7185;
  }
  header {
    background: linear-gradient(125deg, #020617 0%, #1e3a5f 40%, #1d4ed8 72%, #0f766e 100%);
    box-shadow: 0 12px 40px rgba(0, 0, 0, 0.35);
  }
  nav {
    background: linear-gradient(180deg, #111c2f 0%, #0f172a 100%);
    border-left-color: var(--c-teal);
  }
  nav a:hover { color: #bfdbfe; }
  section > h2 {
    border-image: linear-gradient(90deg, var(--c-blue), var(--c-teal)) 1;
  }
  section > h3 { color: #93c5fd; }
  .table th {
    background: linear-gradient(180deg, #1a2744 0%, #152238 100%);
    color: #c7d2fe;
  }
  .table tr:hover td { background: #152238; }
  .placeholder-media,
  .doc-note,
  .power-cycle-wrap,
  .mermaid {
    background: #0d162a;
  }
  section blockquote {
    background: linear-gradient(90deg, rgba(20, 184, 166, 0.12), #0d162a);
    border-left-color: var(--c-teal);
  }
  section blockquote p {
    color: var(--muted);
  }
  .placeholder-media {
    border-color: rgba(45, 212, 191, 0.35);
    background: linear-gradient(145deg, #0c1e2d 0%, #0d162a 100%);
  }
  .doc-note {
    background: linear-gradient(90deg, rgba(20, 184, 166, 0.12), #0d162a);
    border-left-color: var(--c-teal);
  }
  .accordion-item summary {
    background: linear-gradient(90deg, #1a2744 0%, #152238 100%) !important;
  }
  .accordion-item:nth-child(3n+1) summary {
    background: linear-gradient(90deg, #1e3a5f 0%, #152238 100%) !important;
  }
  .accordion-item:nth-child(3n+2) summary {
    background: linear-gradient(90deg, #134e4a 0%, #0f172a 100%) !important;
  }
  .accordion-item:nth-child(3n) summary {
    background: linear-gradient(90deg, #312e81 0%, #1e1b4b 100%) !important;
  }
  .card-link {
    background: linear-gradient(165deg, #111c2f 0%, #0f172a 100%);
  }
  .card-link strong { color: #e2e8f0; }
  .card-link:hover {
    box-shadow: 0 12px 32px rgba(0, 0, 0, 0.5);
  }
  .footer {
    background: linear-gradient(180deg, transparent, rgba(59, 130, 246, 0.06));
  }
}