diff --git a/site/components/Header.astro b/site/components/Header.astro
index fc507bd39..6713eea50 100644
--- a/site/components/Header.astro
+++ b/site/components/Header.astro
@@ -38,10 +38,13 @@ const { activeNav } = Astro.props;
       type="button"
       class="theme-toggle"
       data-theme-toggle
-      aria-label="Switch to light mode"
-      title="Light mode"
-      data-next-theme="light"
+      aria-label="Theme: auto, matching your system. Click to switch to light mode."
+      title="Theme: auto (matches system)"
     >
+      <svg class="theme-toggle-icon--auto" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" aria-hidden="true">
+        <circle cx="12" cy="12" r="9"/>
+        <path d="M12 3a9 9 0 0 1 0 18z" fill="currentColor" stroke="none"/>
+      </svg>
       <svg class="theme-toggle-icon--light" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" aria-hidden="true">
         <circle cx="12" cy="12" r="4.5"/>
         <path d="M12 2v2M12 20v2M4.22 4.22l1.42 1.42M18.36 18.36l1.42 1.42M2 12h2M20 12h2M4.22 19.78l1.42-1.42M18.36 5.64l1.42-1.42"/>
diff --git a/site/layouts/Base.astro b/site/layouts/Base.astro
index 3b068ddf9..07b913042 100644
--- a/site/layouts/Base.astro
+++ b/site/layouts/Base.astro
@@ -69,12 +69,15 @@ const resolvedOgDescription = ogDescription || description;
       try {
         var t = localStorage.getItem('impeccable-theme');
         var root = document.documentElement;
-        if (t === 'light') {
-          root.classList.add('light');
+        var pref = (t === 'light' || t === 'dark') ? t : 'auto';
+        var resolved = pref === 'auto'
+          ? (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark')
+          : pref;
+        root.classList.add(resolved);
+        root.setAttribute('data-theme-pref', pref);
+        if (resolved === 'light') {
           var meta = document.querySelector('meta[name="theme-color"]');
           if (meta) meta.setAttribute('content', '#f7f4ef');
-        } else {
-          root.classList.add('dark');
         }
       } catch (e) {}
     })();
diff --git a/site/pages/design-system/index.astro b/site/pages/design-system/index.astro
index 88fbf3ac4..a114574d9 100644
--- a/site/pages/design-system/index.astro
+++ b/site/pages/design-system/index.astro
@@ -45,10 +45,13 @@ import '../../styles/design-system.css';
         type="button"
         class="theme-toggle"
         data-theme-toggle
-        aria-label="Switch to light mode"
-        title="Light mode"
-        data-next-theme="light"
+        aria-label="Theme: auto, matching your system. Click to switch to light mode."
+        title="Theme: auto (matches system)"
       >
+        <svg class="theme-toggle-icon--auto" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" aria-hidden="true">
+          <circle cx="12" cy="12" r="9"/>
+          <path d="M12 3a9 9 0 0 1 0 18z" fill="currentColor" stroke="none"/>
+        </svg>
         <svg class="theme-toggle-icon--light" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.75" aria-hidden="true">
           <circle cx="12" cy="12" r="4.5"/>
           <path d="M12 2v2M12 20v2M4.22 4.22l1.42 1.42M18.36 18.36l1.42 1.42M2 12h2M20 12h2M4.22 19.78l1.42-1.42M18.36 5.64l1.42-1.42"/>
diff --git a/site/scripts/utils/theme.js b/site/scripts/utils/theme.js
index 82f344e31..2be271045 100644
--- a/site/scripts/utils/theme.js
+++ b/site/scripts/utils/theme.js
@@ -1,21 +1,48 @@
-// Theme toggle — light / dark with localStorage persistence.
+// Theme switcher — three-way: auto / light / dark with localStorage persistence.
+// "auto" (the default) inherits from the OS via prefers-color-scheme and is only
+// overridden once the user clicks the toggle. Clicking cycles auto → light → dark → auto.
 
 const STORAGE_KEY = 'impeccable-theme';
 
-export function getStoredTheme() {
-  return localStorage.getItem(STORAGE_KEY);
+export function getStoredPref() {
+  const v = localStorage.getItem(STORAGE_KEY);
+  return v === 'light' || v === 'dark' ? v : 'auto';
 }
 
-export function setStoredTheme(theme) {
-  localStorage.setItem(STORAGE_KEY, theme);
+export function setStoredPref(pref) {
+  if (pref === 'auto') localStorage.removeItem(STORAGE_KEY);
+  else localStorage.setItem(STORAGE_KEY, pref);
 }
 
-export function applyTheme(theme) {
+function systemTheme() {
+  return window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches
+    ? 'light'
+    : 'dark';
+}
+
+export function resolveTheme(pref) {
+  return pref === 'auto' ? systemTheme() : pref;
+}
+
+const LABELS = {
+  auto: 'Theme: auto, matching your system. Click to switch to light mode.',
+  light: 'Theme: light. Click to switch to dark mode.',
+  dark: 'Theme: dark. Click to switch back to auto.',
+};
+
+const TITLES = {
+  auto: 'Theme: auto (matches system)',
+  light: 'Theme: light',
+  dark: 'Theme: dark',
+};
+
+export function applyTheme(pref) {
   const html = document.documentElement;
-  const resolved = theme === 'light' ? 'light' : 'dark';
+  const resolved = resolveTheme(pref);
 
   html.classList.remove('light', 'dark');
   html.classList.add(resolved);
+  html.setAttribute('data-theme-pref', pref);
 
   const meta = document.querySelector('meta[name="theme-color"]');
   if (meta) {
@@ -23,21 +50,32 @@ export function applyTheme(theme) {
   }
 
   document.querySelectorAll('[data-theme-toggle]').forEach((btn) => {
-    const next = resolved === 'light' ? 'dark' : 'light';
-    btn.setAttribute('aria-label', resolved === 'light' ? 'Switch to dark mode' : 'Switch to light mode');
-    btn.setAttribute('title', resolved === 'light' ? 'Dark mode' : 'Light mode');
-    btn.dataset.nextTheme = next;
+    btn.setAttribute('aria-label', LABELS[pref]);
+    btn.setAttribute('title', TITLES[pref]);
   });
 }
 
+function nextPref(pref) {
+  return pref === 'auto' ? 'light' : pref === 'light' ? 'dark' : 'auto';
+}
+
 export function initThemeToggle() {
-  const stored = getStoredTheme();
-  applyTheme(stored === 'light' ? 'light' : 'dark');
+  applyTheme(getStoredPref());
+
+  // While in auto, follow the OS as it flips.
+  if (window.matchMedia) {
+    const mq = window.matchMedia('(prefers-color-scheme: light)');
+    const onChange = () => {
+      if (getStoredPref() === 'auto') applyTheme('auto');
+    };
+    if (mq.addEventListener) mq.addEventListener('change', onChange);
+    else if (mq.addListener) mq.addListener(onChange);
+  }
 
   document.querySelectorAll('[data-theme-toggle]').forEach((btn) => {
     btn.addEventListener('click', () => {
-      const next = btn.dataset.nextTheme || (document.documentElement.classList.contains('light') ? 'dark' : 'light');
-      setStoredTheme(next);
+      const next = nextPref(getStoredPref());
+      setStoredPref(next);
       applyTheme(next);
     });
   });
diff --git a/site/styles/light-mode.css b/site/styles/light-mode.css
index 041f1873f..0aa78cce7 100644
--- a/site/styles/light-mode.css
+++ b/site/styles/light-mode.css
@@ -44,15 +44,18 @@
   display: block;
 }
 
+/* Three-way switcher: show the icon for the active preference (auto / light /
+   dark), not the resolved theme. Keyed on data-theme-pref so the icon reflects
+   that "auto" is its own state rather than collapsing into whatever the OS picked. */
+.theme-toggle .theme-toggle-icon--auto,
+.theme-toggle .theme-toggle-icon--light,
 .theme-toggle .theme-toggle-icon--dark {
   display: none;
 }
 
-html.light .theme-toggle .theme-toggle-icon--light {
-  display: none;
-}
-
-html.light .theme-toggle .theme-toggle-icon--dark {
+html[data-theme-pref='auto'] .theme-toggle .theme-toggle-icon--auto,
+html[data-theme-pref='light'] .theme-toggle .theme-toggle-icon--light,
+html[data-theme-pref='dark'] .theme-toggle .theme-toggle-icon--dark {
   display: block;
 }
 
@@ -231,19 +234,27 @@ html.light :is(
   color: var(--ks-kinpaku-ink);
 }
 
-/* Command palette (magazine spread) — the title, active command, and category
-   kicker sit on a near-white card (.magazine-container), not the warm paper, so
-   pristine kinpaku reads with enough contrast. Override the muddier kinpaku-ink
-   the shared eyebrow rule above assigns. Scoped under .magazine-container so it
-   only touches the palette (and so its specificity clears the eyebrow rule's
-   :is(), which is inflated to 4 classes by the .fisheye-item.is-active arg).
-   Other eyebrows stay on kinpaku-ink for paper legibility. */
+/* Command palette (magazine spread). The command NAME — big title + active
+   list item — is the brand gold; on the near-white card (.magazine-container,
+   not warm paper) pristine kinpaku reads fine, so override the muddier
+   kinpaku-ink the shared eyebrow rule above assigns. The category kicker is
+   handled separately below (it takes the section accent, not flat gold).
+   Scoped under .magazine-container so it only touches the palette and clears
+   the eyebrow rule's :is() specificity (inflated to 4 classes by its
+   .fisheye-item.is-active arg). Other eyebrows keep kinpaku-ink for paper. */
 html.light .home-kinpaku .magazine-container .spread-command-name,
-html.light .home-kinpaku .magazine-container .spread-category-label,
 html.light .home-kinpaku .magazine-container .fisheye-item.is-active {
   color: var(--ks-kinpaku);
 }
 
+/* The category kicker takes the section's own accent (--spread-accent), like
+   the slash and the divider, so each section reads in its identity color
+   (gold for create/refine/simplify, verdigris for evaluate/harden, neutral for
+   system) rather than a flat gold. Dark mode gets the same via home-kinpaku.css. */
+html.light .home-kinpaku .magazine-container .spread-category-label {
+  color: var(--spread-accent);
+}
+
 /* ============================================================
    Homepage — hero, scroll glass header, section shells
    ============================================================ */