What makes you say this specifically about darktable? Sure we need more Mac and windows people, but if all you want to program strictly new features, you should do that, and there is room to do that. @elstoc is a developer and writes a lot of the documentation.
Or you can tackle any bug. If you want to make a large change, I’d open an issue and ask about it first.
You can rotate it to any arbitrary rotation. Right click the slider and enter a value.
Are you talking about something specific, or are you just generalizing?
It would be your effort, since you have the issue. So you should tell us if you want to figure it out, and people will point you in the right direction, or not bother with it.
I’d like to echo @paperdigits’ comments here. I started as a developer on darktable 2 years ago because there were a few niggling bugs that weren’t getting any attention. I knew nothing of the codebase and hadn’t written any C code for 20 years. I’ve found the project to be, though occasionally (and reasonably) sceptical about new ideas, at the same time very welcome to new contributors.
Find a bug or enhancement and raise a pull request. In the end if you have a very specific use case that’s causing you a problem but is not affecting the developers or is hard to investigate/fix, the best way to resolve it is to do it yourself (if you can).
There are plenty of ways to contribute if you don’t want to get into writing the code (testing, documentation, comments on feature requests).
If you need help understanding the code, you could join us on the #darktable IRC channel on oftc.