Add support for light / dark mode

[driusan]

We should support both light mode and dark mode based on the phone's system preferences.

The easiest way to do this is probably with useTheme from the React Navigation package. (If we go that route, it might be a good opportunity to make the page navigation use React Navigation in general.) If all we care about is the theming and not the navigation , it's possible that using react-native-paper instead of React Navigation would have prettier material design based theming options.