:root {
  --bg: #0e1116;
  --bg-elev: #171d26;
  --text: #e8eef5;
  --text-dim: #9aa7b4;
  --muted: #667080;
  --line: #2b333f;
  --seg-line: rgba(255, 255, 255, 0.16);
  --node-border: rgba(255, 255, 255, 0.24);
  --accent: #22d3ee;
  --accent2: #a78bfa;
  --rail-dot: #333c48;
  --panel-glow: rgba(34, 211, 238, 0.10);
}
:root[data-theme="light"] {
  --bg: #f6f7f9;
  --bg-elev: #ffffff;
  --text: #101623;
  --text-dim: #47536a;
  --muted: #94a0b3;
  --line: #d3d9e2;
  --seg-line: rgba(0, 0, 0, 0.14);
  --node-border: rgba(0, 0, 0, 0.26);
  --accent: #0891b2;
  --accent2: #7c3aed;
  --rail-dot: #cdd5e0;
  --panel-glow: rgba(8, 145, 178, 0.08);
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  color: var(--text);
  background: var(--bg);
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
  transition: background 0.4s ease, color 0.4s ease;
}

/* ---- fixed graphic ---- */
.graphic {
  position: fixed;
  top: 0;
  right: 0;
  width: 46vw;
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  pointer-events: none;
}
.graphic::before {
  content: "";
  position: absolute;
  width: 70%;
  height: 60%;
  background: radial-gradient(ellipse at center, var(--panel-glow), transparent 70%);
  filter: blur(20px);
}
#net { width: 100%; height: 94vh; position: relative; }

/* ---- scrolling story ---- */
.story {
  width: 54vw;
  padding: 0 5vw 0 6vw;
}
.step {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  padding: 8vh 0;
  opacity: 0.22;
  transform: translateY(8px);
  transition: opacity 0.5s ease, transform 0.5s ease;
}
.step.active { opacity: 1; transform: none; }

.kicker {
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.18em;
  font-size: 0.78rem;
  font-weight: 600;
  margin: 0 0 0.9rem;
}
h1 {
  font-size: clamp(2rem, 4.5vw, 3.4rem);
  font-weight: 600;
  letter-spacing: -0.02em;
  line-height: 1.08;
  overflow-wrap: break-word;
  margin: 0 0 0.5em;
  background: linear-gradient(120deg, var(--accent), var(--accent2));
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}
h2 {
  font-size: clamp(1.7rem, 3vw, 2.1rem);
  font-weight: 600;
  letter-spacing: -0.015em;
  margin: 0 0 0.6em;
}
p  { font-size: 1.06rem; margin: 0 0 1em; max-width: 32em; color: var(--text-dim); }
ul { font-size: 1.06rem; max-width: 32em; padding-left: 1.3em; color: var(--text-dim); }
li { margin: 0.35em 0; }
strong { color: var(--text); font-weight: 600; }
em { font-style: italic; color: var(--text); }
.note { font-size: 0.9rem; color: var(--muted); }

/* hero */
.hero .tagline { font-size: 1.2rem; color: var(--text-dim); max-width: 28em; }
.scroll-cue {
  margin-top: 2.6rem;
  display: flex; flex-direction: column; gap: 10px; align-items: flex-start;
  color: var(--muted); font-size: 0.82rem; letter-spacing: 0.05em;
}
.chev {
  width: 12px; height: 12px;
  border-right: 2px solid var(--accent);
  border-bottom: 2px solid var(--accent);
  transform: rotate(45deg);
  animation: bounce 1.5s ease-in-out infinite;
}
@keyframes bounce {
  0%, 100% { transform: rotate(45deg) translate(0, 0); opacity: 0.4; }
  50%      { transform: rotate(45deg) translate(3px, 3px); opacity: 1; }
}

/* ---- math ---- */
.katex { color: var(--text); }
.formula { margin: 0.9em 0 1.2em; overflow-x: auto; overflow-y: hidden; }
.formula .katex-display { text-align: left; margin: 0.3em 0; }
.formula .katex { font-size: clamp(1rem, 1.9vw, 1.5em); }
.imath .katex { font-size: 1.04em; }

/* ---- SVG chrome (theme-aware via CSS vars) ---- */
#net rect { transition: fill 0.45s ease; }
#net .edge { stroke: var(--line); stroke-width: 1.5; fill: none; }
#net .nseg { stroke: var(--seg-line); stroke-width: 1; }
#net .nborder { stroke: var(--node-border); stroke-width: 1.2; fill: none; }
#net .fc { fill: var(--bg-elev); stroke: var(--node-border); stroke-width: 1; }
#net .ecirc { fill: var(--bg-elev); stroke: var(--node-border); stroke-width: 1.3; }

#net .flow {
  stroke: var(--accent);
  stroke-width: 2.4;
  fill: none;
  stroke-linecap: round;
  stroke-dasharray: 2.5 9;
  filter: drop-shadow(0 0 3px var(--accent));
  animation: flow 0.9s linear infinite;
}
#net .flow.rev { animation: flowrev 0.9s linear infinite; }
@keyframes flow { to { stroke-dashoffset: -11.5; } }
@keyframes flowrev { to { stroke-dashoffset: 11.5; } }

.blink {
  stroke: var(--accent) !important;
  stroke-width: 2.6 !important;
  filter: drop-shadow(0 0 4px var(--accent));
  animation: blinkpulse 0.95s ease-in-out infinite;
}
@keyframes blinkpulse { 0%, 100% { stroke-opacity: 1; } 50% { stroke-opacity: 0.25; } }

/* ---- top progress bar ---- */
.progress-top {
  position: fixed; top: 0; left: 0; height: 3px; width: 0;
  background: linear-gradient(90deg, var(--accent), var(--accent2));
  box-shadow: 0 0 10px var(--accent);
  z-index: 50; transition: width 0.12s linear;
}

/* ---- theme toggle ---- */
.theme-toggle {
  position: fixed; top: 14px; right: 18px; z-index: 50;
  background: var(--bg-elev); color: var(--text);
  border: 1px solid var(--node-border); border-radius: 20px;
  padding: 6px 14px; font-size: 13px; font-family: inherit; cursor: pointer;
  transition: border-color 0.2s ease, transform 0.1s ease;
}
.theme-toggle:hover { border-color: var(--accent); }
.theme-toggle:active { transform: scale(0.96); }

/* ---- side rail ---- */
.rail {
  position: fixed; left: 16px; top: 50%; transform: translateY(-50%);
  display: flex; flex-direction: column; gap: 9px; z-index: 40;
}
.rail button {
  width: 9px; height: 9px; padding: 0; border: none; border-radius: 50%;
  background: var(--rail-dot); cursor: pointer; position: relative;
  transition: transform 0.25s ease, background 0.25s ease, box-shadow 0.25s ease;
}
.rail button:hover { background: var(--accent2); }
.rail button.active {
  background: var(--accent); box-shadow: 0 0 9px var(--accent); transform: scale(1.55);
}
.rail button::after {
  content: attr(data-title);
  position: absolute; left: 18px; top: 50%; transform: translateY(-50%);
  white-space: nowrap; background: var(--bg-elev); color: var(--text);
  font-size: 11px; padding: 3px 9px; border-radius: 6px;
  border: 1px solid var(--node-border);
  opacity: 0; pointer-events: none; transition: opacity 0.18s ease;
}
.rail button:hover::after { opacity: 1; }

/* ---- footer ---- */
.site-footer {
  padding: 70px 0 90px; margin-top: 30px;
  border-top: 1px solid var(--line);
  color: var(--muted); font-size: 0.9rem;
}
.site-footer .credit { color: var(--text-dim); }
.site-footer strong { color: var(--accent); }

/* ---- small screens: stack, hide rail ---- */
@media (max-width: 720px) {
  .graphic { position: relative; width: 100%; height: 56vh; }
  .graphic::before { display: none; }
  .story { width: 100%; padding: 0 6vw; }
  #net { height: 52vh; }
  .rail { display: none; }
}