CSS is a subset of LESS so all existing MediaWiki:Common.css should just work™.

Migrating to less will allow us to organise these large files a lot better by making use of nesting and will lead to a lot of CSS improvements, e.g. removing dead CSS just like when Vector's CSS was ported to less.

I suggest we just do this and no longer support standard CSS pages for this kind of thing. We need to organise these pages better.