Implement Dark Mode

Summary:

Implement Dark Mode

how is it useful:

This is something. many other social media apps have done. it would be nice to have a design for dark mode done. we could ask a dev to make a 1st pass in a git branch and set a feature flag switch in test mode menu, look at it, then vlada could propose changes and improvements. as we do this, on android, we could implement material me as well, flutter supoorta this well, and it would have a nice feel for android users of more recent phones.