TL:DR Depends on the style/theme, author, their licensing choices and whether or not they like to host the source code publicly or its just in a folder on their PC. If the information about a public repo is not on style/theme page, contact the Authors of the styles you want to contribute to and find out.
Many Styles here dont have a repository to contribute to, which is sort of frustrating to contribute fixes, but usually if you contact the style theme author via private message or via style feedback you may get some more information.
Comments
To my theme you can contribute https://github.com/the-j0k3r/userstyles-inspired-dark repository.
There are quite a few themes hosted at GitHub and their repositories are marked on the style page (like mine)
For instance Github-Dark https://userstyles.org/styles/37035/github-dark has its own repo.
Many Styles here dont have a repository to contribute to, which is sort of frustrating to contribute fixes, but usually if you contact the style theme author via private message or via style feedback you may get some more information.
Regards.