Subscribe to native theme changes

ts/background.ts

@@ -2284,6 +2284,10 @@ export async function startApp(): Promise<void> {
     }
   }
 
+  window.subscribeToSystemThemeChange(() => {
+    onChangeTheme();
+  });
+
   const FIVE_MINUTES = 5 * 60 * 1000;
 
   // Note: once this function returns, there still might be messages being processed on

ts/window.d.ts

@@ -166,6 +166,8 @@ declare global {
 
     WhatIsThis: WhatIsThis;
 
+    subscribeToSystemThemeChange: (fn: () => void) => void;
+
     registerScreenShareControllerRenderer: (
       f: (
         component: typeof CallingScreenSharingController,