Fixed theme switcher bfcache handling.

Reloaded the theme setting from the cookie when the browser restores page state
from the backward/forward cache. This fixes a problem where a theme change on
one page appeared to be reverted when navigating via history buttons.

Author: medmunds

djangoproject/static/js/mod/switch-dark-mode.js

@@ -95,6 +95,13 @@ document.addEventListener('DOMContentLoaded', function () {
   setupThemeToggle();
 });
 
+window.addEventListener('pageshow', function (e) {
+  if (e.persisted) {
+    // Re-initialize from cookie in case theme was changed on another page.
+    initTheme();
+  }
+});
+
 prefersDarkMediaQuery.addEventListener('change', (e) => {
   prefersDark = e.matches;
 });