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/app/src/init.tsx

@@ -215,7 +215,7 @@ function init() {
     {name: 'loadTheme', fireImmediately: false, throttleMs: 500},
     (state) => {
       const theme = state.settingsState.darkMode;
-      let shouldUseDarkMode = remote.nativeTheme.shouldUseDarkColors;
+      let shouldUseDarkMode = false;
       if (theme === 'dark') {
         shouldUseDarkMode = true;
       } else if (theme === 'light') {