For CSS, there is no need for them to be approved. You need to put them into wiki.fandom.com/wiki/MediaWiki:Wikia.css to get them showing up on your wiki.
Yes. All JS applied site-wide, regardless of scope/action/intent, must be reviewed and approved by staff. No nexceptions. ImportJS appears to provide an exception but actually doesn't. While ImportJS itself is not reviewed, each script that is imported is.