Fix startup error when loading theme

Summary: During testing, observed that Flipper always starts with an exception if `darkMode` is `true` or `false` (the old format) during startup, and this exception keeps on happening until the setting is changed. The root cause is that the setting is treated verbatim since D30666966 (9a4d94c971), trigger a very confusing Electron error. This diff addresses that.

Reviewed By: timur-valiev

Differential Revision: D30806453

fbshipit-source-id: fc7bbdda4e8bdf2dc4e3ca7ab1b05984c9406c68

desktop/static/setup.ts

@@ -56,6 +56,12 @@ export default function setup(argv: any) {
   // Non-persistent CLI arguments.
   config = {
     ...config,
+    darkMode:
+      typeof config.darkMode === 'boolean'
+        ? config.darkMode // normalise darkmode from old format
+          ? 'dark'
+          : 'light'
+        : config.darkMode,
     updaterEnabled: argv.updater,
     launcherEnabled: argv.launcher,
     launcherMsg: argv.launcherMsg,