I’ve been maintaining Theseus to make sure that it continues to work well as Brackets evolves (for example, I pushed some fixes recently, and I’ll be reviewing your pull request shortly). I’ve also been working with some people privately who are interested in making big changes to Theseus. I haven’t been able to make the time to do significant work myself, though.
If you know of any bugs that don’t have associated issues on GitHub, it would be great if you could add them. To the best of my knowledge, all open bugs are either waiting for further comment from the people who opened them, or are in my local todo list. I’ve fixed the nastiest ones as quickly as possible, and made steady progress on the rest, but I have to balance the work with starting a company that has nothing to do with Theseus. :)