Dynamic colors cause a Main Thread Checker violation

[tom-un]

The Dynamic color feature, which is to specify a color via { dynamic: { light: '', dark: '' } } is causing a Main Thread checker violation at runtime.

Steps To Reproduce

1. Debug RNTester in Xcode
2. In the console a Main Thread check failure will appear in RCTDynamicColor.m when calling [NSApp effectiveAppearance]

Its harmless (now) but needs to be refactored in a way that [NSApp effectiveAppearance] doesn't get called on a background thread.