Add support for theme switching

[kikisaints]

Within the settings page, we should have the ability for the user to change their theme default from dark to light.

This may also mean saving that settings locally somehow so the user doesn't have to do this every time. Perhaps through AsyncStorage?

[chrisglein]

@chiaramooney At least from when I checked that latest in main this week it seems like this is in? Safe to close as of #28?

[chiaramooney]

At least from when I checked that latest in main this week it seems like this is in? Safe to close as of #28?

@chrisglein Right now the app supports theme switching the color of the code snippets but not the entire app, so still some work to be done here

[kmelmon]

Currently we have the ability to theme code snippets, the rest is pending