View Transition Theme Mode

:root {
  --bg: #070d19;
  --panel: #121c31;
  --line: #2a3a57;
  --text: #edf4ff;
  --muted: #bfd0e8;
  --accent: #8fe6ff;
  --accent-strong: #6fd6ff;
}

html[data-theme="paper"] {
  --bg: #f3f0e8;
  --panel: #fffdf8;
  --line: #d9d1c4;
  --text: #22273b;
  --muted: #5c6170;
  --accent: #305b8f;
  --accent-strong: #1e4e86;
}

html[data-theme="noir"] {
  --bg: #060607;
  --panel: #111214;
  --line: #2b2d31;
  --text: #f5f7fb;
  --muted: #c9ced6;
  --accent: #b5d6ff;
  --accent-strong: #89b8ff;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  color: var(--text);
  font-family: "Avenir Next", "Segoe UI", sans-serif;
  background: radial-gradient(
      circle at 12% 10%,
      color-mix(in srgb, var(--accent) 32%, transparent),
      transparent 42%
    ),
    radial-gradient(
      circle at 82% 80%,
      color-mix(in srgb, var(--accent-strong) 28%, transparent),
      transparent 42%
    ), var(--bg);
  transition: background 280ms ease;
}

.topbar {
  padding: 0.85rem 1rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.topbar a {
  color: var(--accent);
  text-decoration: none;
  font-weight: 700;
}

.support {
  margin: 0;
  color: var(--muted);
  font-size: 0.86rem;
}

.support.ok {
  color: #aff0cc;
}
.support.warn {
  color: #ffd6ad;
}

main {
  width: min(1080px, 94%);
  margin: 0 auto 2rem;
}

.hero {
  display: grid;
  gap: 0.6rem;
}
.eyebrow {
  margin: 0;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  font-size: 0.8rem;
  color: var(--accent);
}

h1,
h2,
p {
  margin: 0;
}
.desc {
  color: var(--muted);
  max-width: 66ch;
}

.modes {
  margin-top: 0.8rem;
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
}

.mode-btn {
  border: 1px solid color-mix(in srgb, var(--line) 72%, white);
  border-radius: 999px;
  background: color-mix(in srgb, var(--panel) 88%, transparent);
  color: var(--text);
  padding: 0.35rem 0.7rem;
  cursor: pointer;
}

.mode-btn.active {
  border-color: color-mix(in srgb, var(--accent) 70%, white);
  box-shadow: 0 0 0 1px color-mix(in srgb, var(--accent) 45%, white) inset;
}

.preview {
  margin-top: 1rem;
  display: grid;
  gap: 1rem;
  grid-template-columns: 1.25fr 0.75fr;
}

.panel {
  border: 1px solid var(--line);
  border-radius: 18px;
  background: color-mix(in srgb, var(--panel) 90%, transparent);
  padding: 1rem;
  display: grid;
  gap: 0.7rem;
}

.label {
  width: fit-content;
  border: 1px solid color-mix(in srgb, var(--line) 75%, white);
  border-radius: 999px;
  padding: 0.15rem 0.5rem;
  font-size: 0.75rem;
  color: var(--muted);
}

.title {
  font-size: clamp(1.8rem, 4.8vw, 3.2rem);
  line-height: 0.95;
}

.muted {
  color: var(--muted);
}

.cta {
  width: fit-content;
  border: 0;
  border-radius: 999px;
  padding: 0.6rem 0.9rem;
  font-weight: 700;
  color: #051a26;
  background: linear-gradient(130deg, var(--accent), var(--accent-strong));
  cursor: pointer;
}

.side {
  align-content: center;
  gap: 0.75rem;
}

.swatch {
  border-radius: 12px;
  min-height: 88px;
  border: 1px solid var(--line);
}

.swatch.a {
  background: linear-gradient(
    130deg,
    var(--accent),
    color-mix(in srgb, var(--accent-strong) 65%, black)
  );
}
.swatch.b {
  background: linear-gradient(
    130deg,
    color-mix(in srgb, var(--panel) 20%, var(--accent)),
    color-mix(in srgb, var(--accent-strong) 75%, black)
  );
}
.swatch.c {
  background: linear-gradient(
    130deg,
    color-mix(in srgb, var(--accent-strong) 80%, black),
    color-mix(in srgb, var(--accent) 30%, var(--bg))
  );
}

::view-transition-old(root),
::view-transition-new(root) {
  animation-duration: 360ms;
  animation-timing-function: cubic-bezier(0.2, 0.8, 0.2, 1);
}

@media (max-width: 880px) {
  .preview {
    grid-template-columns: 1fr;
  }
}

if (!window.MotionPreference) {
  const __mql = window.matchMedia("(prefers-reduced-motion: reduce)");
  const __listeners = new Set();

  const MotionPreference = {
    prefersReducedMotion() {
      return __mql.matches;
    },
    setOverride(value) {
      const reduced = Boolean(value);
      document.documentElement.classList.toggle("reduced-motion", reduced);
      window.dispatchEvent(new CustomEvent("motion-preference", { detail: { reduced } }));
      for (const listener of __listeners) {
        try {
          listener({ reduced, override: reduced, systemReduced: __mql.matches });
        } catch {}
      }
    },
    onChange(listener) {
      __listeners.add(listener);
      try {
        listener({
          reduced: __mql.matches,
          override: null,
          systemReduced: __mql.matches,
        });
      } catch {}
      return () => __listeners.delete(listener);
    },
    getState() {
      return { reduced: __mql.matches, override: null, systemReduced: __mql.matches };
    },
  };

  window.MotionPreference = MotionPreference;
}

const themes = ["neon", "paper", "noir"];
const root = document.documentElement;
const modes = document.getElementById("modes");
const support = document.getElementById("support");
const reduced = window.MotionPreference.prefersReducedMotion();

const state = { theme: root.dataset.theme || "neon" };

function applyTheme(next) {
  root.dataset.theme = next;
  state.theme = next;
  renderButtons();
}

function transition(update) {
  if (!reduced && document.startViewTransition) {
    document.startViewTransition(update);
  } else {
    update();
  }
}

function renderButtons() {
  modes.innerHTML = "";
  themes.forEach((theme) => {
    const btn = document.createElement("button");
    btn.className = `mode-btn ${state.theme === theme ? "active" : ""}`;
    btn.textContent = theme;
    btn.addEventListener("click", () => {
      if (theme === state.theme) return;
      transition(() => applyTheme(theme));
    });
    modes.appendChild(btn);
  });
}

function renderSupport() {
  if (document.startViewTransition && !reduced) {
    support.textContent = "Animated mode switch enabled with shared title/CTA continuity.";
    support.classList.add("ok");
  } else {
    support.textContent = "Fallback mode: theme updates are instant and fully usable.";
    support.classList.add("warn");
  }
}

renderButtons();
renderSupport();

<!doctype html>
<html lang="en" data-theme="neon">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Demo 18 - Theme Mode Transition</title>
    <link rel="stylesheet" href="style.css" />
  </head>
  <body>
    <header class="topbar">
      <a href="../">Back to demos</a>
      <p id="support" class="support"></p>
    </header>

    <main>
      <section class="hero">
        <p class="eyebrow">Demo 18</p>
        <h1>Theme/Mode Shared-Element Transition</h1>
        <p class="desc">Switch between visual modes while preserving hero identity and CTA hierarchy.</p>

        <div class="modes" id="modes"></div>
      </section>

      <section class="preview">
        <article class="panel">
          <p class="label">Brand Core</p>
          <h2 class="title" style="view-transition-name:brand-title">Nova Motion</h2>
          <p class="muted">A shared heading stays continuous while global palette and texture shift.</p>
          <button class="cta" style="view-transition-name:brand-cta">Get Early Access</button>
        </article>

        <article class="panel side">
          <div class="swatch a"></div>
          <div class="swatch b"></div>
          <div class="swatch c"></div>
        </article>
      </section>
    </main>

    <script src="script.js"></script>
      </body>
</html>