Fix dark mode startup error

Summary:
Changelog: Fixed startup error when loading default theme

The theme defaulted to `auto`, instead of `system`, causing an startup exception. Fixed by adding types and having a more defensive startup check.

Fixes https://github.com/facebook/flipper/issues/2835

Reviewed By: jknoxville

Differential Revision: D30866648

fbshipit-source-id: 83b6d9fc235eaa0a7e959df4276d3f378eed7d1f

desktop/static/main.ts

@@ -109,7 +109,9 @@ if (argv['disable-gpu'] || process.env.FLIPPER_DISABLE_GPU === '1') {
 }
 
 process.env.CONFIG = JSON.stringify(config);
-nativeTheme.themeSource = config.darkMode || 'light';
+nativeTheme.themeSource = ['light', 'dark', 'system'].includes(config.darkMode)
+  ? config.darkMode
+  : 'light';
 
 // possible reference to main app window
 let win: BrowserWindow;