You can edit the wiki's CSS in Wikia.css, but you need special rights from Fandom to edit JS

Why?

I really can't, the CSS page said I'm not allowed to execute the action I've requested.

Have you tried to promote yourself? I have promoted myself so I can get more rights, although it is not necessary

How? I'm a bureaucrat and sysop (default) and I made myself a rollback

Can you give me a link to your wiki?