I’m (likely) the only developer left still taking professional contracts to build custom stylesheets for old Reddit, the version of Reddit prior to the 2018 redesign.
Much of the info once available has succumbed to link rot, so I’ve covered what I can in a modern article here: https://www.reddit.com/r/csshelp/comments/1s4p96w/a_modern_guide_to_old_reddit_customisation_and/