Ability to change board and piece themes

[clarkerubber]

Personally (at least to start with), I'd suggest that the default board and piece theme be the same as the lichess website.

These are small details, but lichess has a minimalistic image to maintain. I think that the app will be better received if it follows on with the same design cues as the website.

I think I saw a while ago that a white on white cburnett splash screen was going to be used on the application originally. I'm not sure what happened to these images or if they'll make a return, but they were much more appropriate than the current splash screen.