Add Dark Mode support

[ZachOrr]

There's a lot of moving parts to supporting dark mode in our React Native stack

It looks like we can use the Appearance module (example here) to respond to theme changes. We'll also need to update semantic colors where possible.

These features haven't made it in to a RN release yet

[ZachOrr]

We might be able to use https://github.com/expo/react-native-appearance as a stop-gap until the Appearance changes hit master