Start application in the right theme

Summary:
Changelog: Improved dark mode support!

This diff makes sure that the dark mode preference is written to the Flipper config, and applied during startup, so that the ugly light/dark flash when starting Flipper in dark mode disappears

Reviewed By: passy

Differential Revision: D29436059

fbshipit-source-id: 0f762149848298512026fbd216d9a9e0bf4276db

desktop/static/setup.ts

@@ -24,6 +24,7 @@ export type Config = {
   launcherMsg?: string | undefined;
   updaterEnabled?: boolean;
   launcherEnabled?: boolean;
+  darkMode?: boolean;
 };
 
 export default function setup(argv: any) {