Hook up theme and clear responses

src-web/hooks/useTheme.ts

@@ -0,0 +1,14 @@
+import { useEffect } from 'react';
+import { setTheme, subscribeToPreferredThemeChange, toggleTheme } from '../lib/theme';
+
+export default function useTheme(subscribeToChanges = true): { toggleTheme: () => void } {
+  useEffect(() => {
+    if (!subscribeToChanges) return;
+    const unsub = subscribeToPreferredThemeChange(setTheme);
+    return unsub;
+  }, [subscribeToChanges]);
+
+  return {
+    toggleTheme: toggleTheme,
+  };
+}