As far as general releases go, I'll try to keep all the different versions in sync (GitHub, Google Play, TestFlight) to minimize the amount of confusion.
I would like to give a big thanks to @CTalvio for helping with the graphics and screenshots that you see on the listing page, and others for contributing to the overall project! This would not be possible without all their help. Thunder is a community-driven project, and all your contributions towards the project help shape the way it is. If you are interested in contributing to Thunder in any way, feel free to reach out!
Anything that doesn't have a github issue yet? I have list of stuff I want to add, but once I'm satisfied, I wont mind adding things to meet the preferences of others.
A lot of these are already being worked on. Some not.
For themes, Thunder is already using a system that would support adding more themes by defining new color palettes. That's definitely planned to eventually be a thing.
But, generally, there's a good few of us now, picking up things from the list of issues and getting them done. We do stuff in the order of what each of us is interested in doing so the fastest way to make a change will always be to do it yourself, but sometimes you want something an existing contributor wants, too, and sometimes you just plain hand out a good idea no-one had thought of yet.