It would be handy if there was some sort of Store/Gallery kind of thing for people to upload their CSS to.
The place should offer a preview as well as insighte in the CSS (and the ability to rate it).
If you want to go a bit further still:
Make it so that custom CSS can be installed as a "plugin" of sorts. (with the local overwrite still)
So you have a list of all your installed CSS packages (with current version, eventual updates, view css etc).
That way people can upload small css snippets (like modifying the look of buttons in one, modifying the look of search results in another, etc) and people can compose them in a way to change Kagi to their liking.