Use dark scrollbar and system elements

Summary:
This diff adds OS support for dark mode (at least on Mac), by making sure system elements are styled according to system dark theme.

For some unknown reason, this does apply to all system dialogs, titlebars etc, but not for scrollbars, so they got custom themed.

Differential Revision: D29436058

fbshipit-source-id: bac649b6cd81f0142c607ed567eeb00f41625041

desktop/app/src/init.tsx

@@ -50,6 +50,7 @@ import {CopyOutlined} from '@ant-design/icons';
 import {clipboard} from 'electron/common';
 import {getVersionString} from './utils/versionString';
 import {PersistGate} from 'redux-persist/integration/react';
+import {ipcRenderer} from 'electron';
 
 if (process.env.NODE_ENV === 'development' && os.platform() === 'darwin') {
   // By default Node.JS has its internal certificate storage and doesn't use
@@ -222,6 +223,7 @@ function init() {
       (
         document.getElementById('flipper-theme-import') as HTMLLinkElement
       ).href = `themes/${theme.dark ? 'dark' : 'light'}.css`;
+      ipcRenderer.send('setTheme', theme.dark ? 'dark' : 'light');
     },
   );
 }