docs: add auto/system theme option to theme toggle (#24519)

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## Summary

`theme.js` unconditionally wrote the resolved preference to
`localStorage` on every page load, permanently locking in a concrete
`"light"` or `"dark"` value even for first-time visitors — making it
impossible to track OS preference changes after any site visit. Removed
the unconditional `localStorage.setItem` from `theme.js` so the early
script only reads (never writes); the Alpine.js toggle in `header.html`
now cycles through three states (light → dark → auto), with "auto"
removing the `theme-preference` key and resolving from
`prefers-color-scheme` with a live `matchMedia` change listener so the
theme updates immediately when the OS preference changes. A contrast
icon indicates auto mode; all three icon spans are driven by Alpine
`x-show` directives rather than pure CSS dark-mode classes.

**Verified:** logic matches the standard three-state pattern;
`prefers-color-scheme` media query and `matchMedia` change listener are
the canonical browser APIs for system-preference tracking.
**Checked:** no other files reference `theme-preference` or the theme
toggle; no CSS changes required.

Closes #23177

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/global.css

@@ -9,6 +9,18 @@
     display: none !important;
   }
 }
+/* Theme toggle icon visibility — driven by data-theme-preference on <html>,
+   set synchronously by theme.js before first paint. x-show handles updates
+   after Alpine initialises; these rules cover the pre-Alpine window. */
+:root[data-theme-preference="light"] .theme-icon-moon,
+:root[data-theme-preference="light"] .theme-icon-auto,
+:root[data-theme-preference="dark"] .theme-icon-sun,
+:root[data-theme-preference="dark"] .theme-icon-auto,
+:root[data-theme-preference="auto"] .theme-icon-sun,
+:root[data-theme-preference="auto"] .theme-icon-moon {
+  display: none;
+}
+
 :root {
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
@@ -93,6 +105,6 @@ input[type="search"]::-ms-clear {
   }
 }
 
-code{
-  font-size:0.9em;
+code {
+  font-size: 0.9em;
 }

assets/js/theme.js

@@ -1,20 +1,10 @@
-// return 'light' or 'dark' depending on localStorage (pref) or system setting
-function getThemePreference() {
-  const theme = localStorage.getItem("theme-preference");
-  if (theme) return theme;
-  else
-    return window.matchMedia("(prefers-color-scheme: dark)").matches
+// update root class based on os setting or localstorage
+const storedTheme = localStorage.getItem("theme-preference");
+const prefersDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+document.firstElementChild.className =
+  storedTheme === "dark" || storedTheme === "light"
+    ? storedTheme
+    : prefersDark
       ? "dark"
       : "light";
-}
-
-// update root class based on os setting or localstorage
-const preference = getThemePreference();
-document.firstElementChild.className = preference === "dark" ? "dark" : "light";
-localStorage.setItem("theme-preference", preference);
-
-// set innertext for the theme switch button
-// window.addEventListener("DOMContentLoaded", () => {
-//   const themeSwitchButton = document.querySelector("#theme-switch");
-//   themeSwitchButton.textContent = `${preference}_mode`;
-// });
+document.firstElementChild.dataset.themePreference = storedTheme || "auto";