Summary:
Fixes#23314 , the change came from #20945 , its purpose is to fix orientation change issue in HelloWord template, but I think it's just a trick, to make rootView's backgroundColor the same as window backgroundColor, the orientation issue seems related to [UIManager setAvailableSize:forRootView:](d2fc19f4aa/React/Modules/RCTUIManager.m (L343)) async layout things.
[iOS] [Fixed] - Revert RCTRootView's backgroundColor to white.
Pull Request resolved: https://github.com/facebook/react-native/pull/23358
Differential Revision: D14030666
Pulled By: cpojer
fbshipit-source-id: 7c9a45f03b87c3be0f2b7c64a3c837c6ae14af3e