Change hardcoded colors to more neutral, and add some basic color schemes

[ddagunts]

Hello, first off, a big thanks for the GrapheneOS project. I am hoping you will consider this PR, or consider a similar/better implementation. The default color scheme in the Messaging app for incoming messages has white text on green background. Green and white is a fairly common color blindness - for me it is difficult to read incoming messages. There are other messaging and SMS apps available, but unfortunately I haven't found an SMS app that works reliably in secondary profiles (well known issues).

This changes the old green defaults to grays, and provides a few color schemes which, while not designer quality, do seem to be easier to read.

Thank you!

[ddagunts]

I can see the code is in a state of modernization, so it's understandable if this change is undesirable and declined. I would ask, in such a case, to consider changing the hue on the message bubbles to a darker green.

Thank you.