We *might* get a secondary mentor from the WMF Design team to support the UI work and also the possibilities of CSS customization options that MediaWiki admins might be interested about. We miss a developer familiar with MediaWiki extension developer. I'm not an expert, but this extension looks relatively simple in terms of backend? It is an interface to manipulate MediaWiki:Common.css.
I actually think (imho) it would be better implemented as a separate resource loader module rather then piggy backing on MediaWiki:Common.css. Otherwise it will get bogged down with trying to parse Common.css to figure out what the current settings are. If its a separate RL module, then the customizations are entirely separate of any junk users added to Common.css (Including syntax errors which I'm sure are common on wikis where the admins don't know css, which is the target audience). The downside of course is users can't as easily "see" the generated css to "learn" how it works under the hood, or make simple modifications to generated css.
Ideas for related bugs that can be assigned to Ioannis as microtasks are also welcome. Just name existing bug reports requiring skills relevant to this project.
Something resource loader related perhaps. Maybe 34339, although that's more backend.