🍎 Add 'dark-mode-changed' event to app api

atom/browser/browser_observer.h

@@ -45,6 +45,8 @@ class BrowserObserver {
   // The browser requests HTTP login.
   virtual void OnLogin(LoginHandler* login_handler) {}
 
+  virtual void OnDarkModeChanged() {}
+
  protected:
   virtual ~BrowserObserver() {}
 };