Let me draw your attention to three GSoC proposals trying to improve the current situation with MediaWiki skins. They didn't come from our list of featured projects,but they point to problems the community has acknowledged, and so far they are responding well to feedback. Your attention and help is welcome.
Sorted by date:
A modern, scalable and attractive skin for MediaWiki, by Jack Phoenix https://www.mediawiki.org/wiki/User:Jack_Phoenix/A_modern,_scalable_and_attr... Isarra and Emufarmers are volunteering as mentors. All the basics seem to be in place, more scrutiny is welcomed.
Separating skins from core MediaWiki, by Bartosz Dziewoński https://www.mediawiki.org/wiki/User:Matma_Rex/Separating_skins_from_core_Med... No mentors assigned yet, Jon Robson is volunteering. More than classical mentors, this proposal needs explicit community consensus, and a clear backing from MatmaRex's fellow Vector maintaners: Trevor Parscal, Roan Kattouw, Timo Tijhof. In fact, one of these (or someone else with +2 amb familiarity with skins) should be a co-mentor to offer a third pair of eyes to the plans and the code reviews.
Frontend for Vector skin CSS customizations, by Ioannis Protonotarios https://www.mediawiki.org/wiki/User:Protnet/Frontend_for_Vector_skin_CSS_cus... No mentors assigned, and nobody is volunteering so far. The proposal is receiving good feedback from known experts in that area. While the previous two candidates are already maintainers in the areas they are proposing to work, and while Ioannis provides details about his experience, he needs mentors happy to evaluate him with microtasks and tough questions.