Please clarify your question; I can't really understand what you are telling

Though for the themes CSS code you can use the selectors ".theme-fandomdesktop-light" and ".theme-fandomdesktop-dark

There is no such role. You can always go to wiki admins and offer your design ideas, but it's up to them to accept or decline.

We have no role for that, unless you are talking about Designing for Fandom itself.