The userchrome.css file support was added in v1.0.176 in this pull request by @devonzuegel.
From the readme
Editor styles can be customized by placing a custom editor style file in the profile directory
~/.config/joplin-desktop/userchrome.css
Unfortunately I don't think there is a method to get a list of all elements, but if you tell me what you're looking to change, I'd be happy to look at the specific elements using a debug version of Joplin and report them back to you!