Profile redesign: Account featured tab (#38529)

Co-authored-by: diondiondion <mail@diondiondion.com>

app/javascript/mastodon/hooks/useTheme.ts

@@ -0,0 +1,23 @@
+import { useEffect, useState } from 'react';
+
+import { isDarkMode } from '../utils/theme';
+
+export function useTheme() {
+  const [darkMode, setDarkMode] = useState(() => isDarkMode());
+
+  useEffect(() => {
+    const mutationObserver = new MutationObserver(() => {
+      setDarkMode(isDarkMode());
+    });
+    mutationObserver.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ['data-color-scheme'],
+    });
+
+    return () => {
+      mutationObserver.disconnect();
+    };
+  }, []);
+
+  return darkMode ? 'dark' : 'light';
+}