Dark mode support for RCTPerfMonitor (#28130)
Dark mode support for RCTPerfMonitor (#28130)

Summary:
Hi There 👋,

While I'm developing on iOS with dark appearance enabled, i notice that `Pref Monitor` view doesn't support dark mode yet, so here is the PR to fix it :)

## Changelog

[iOS] [Fixed] - Fix Pref Monitor in dark appearance
Pull Request resolved: https://github.com/facebook/react-native/pull/28130

Test Plan:
Run any React Native app on iOS > Turn on dark appearance
### Before
![Before](https://user-images.githubusercontent.com/4061838/74872974-8b75b600-535e-11ea-8550-763a598547ec.png)
### After
![After](https://user-images.githubusercontent.com/4061838/74872978-8ca6e300-535e-11ea-9862-4d3014e849f7.png)

Reviewed By: RSNara

Differential Revision: D20080019

Pulled By: PeteTheHeat

fbshipit-source-id: 9365daa3f7193a11760bc1372b8de2c3896def5c

exponentjs/react-native,facebook/react-native,exponent/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,hoangpham95/react-native,hammerandchisel/react-native,hammerandchisel/react-native,pandiaraj44/react-native,arthuralee/react-native,javache/react-native,facebook/react-native,exponent/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponentjs/react-native,exponentjs/react-native,javache/react-native,hammerandchisel/react-native,arthuralee/react-native,myntra/react-native,myntra/react-native,pandiaraj44/react-native,hammerandchisel/react-native,hoangpham95/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,exponentjs/react-native,exponent/react-native,pandiaraj44/react-native,exponentjs/react-native,pandiaraj44/react-native,exponent/react-native,pandiaraj44/react-native,myntra/react-native,facebook/react-native,janicduplessis/react-native,arthuralee/react-native,javache/react-native,exponentjs/react-native,exponentjs/react-native,facebook/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,janicduplessis/react-native,hoangpham95/react-native,hoangpham95/react-native,exponent/react-native,javache/react-native,pandiaraj44/react-native,hoangpham95/react-native,janicduplessis/react-native,facebook/react-native,myntra/react-native,hoangpham95/react-native,facebook/react-native,arthuralee/react-native,myntra/react-native,janicduplessis/react-native,javache/react-native,exponentjs/react-native,hammerandchisel/react-native,myntra/react-native,arthuralee/react-native,pandiaraj44/react-native,javache/react-native,hammerandchisel/react-native,hoangpham95/react-native,hammerandchisel/react-native,exponent/react-native,janicduplessis/react-native,exponent/react-native,myntra/react-native,javache/react-native,exponent/react-native,myntra/react-native