My rule for using LLMs is that anything that's one off is okay. Anything that's more permanent and committed to a repo needs a human review. I strongly suggest you have an understanding of the basics (at least the box model) so that you are competent at reviewing CSS code before using LLM for that.

I've been looking for a good guide on prompting LLMs for CSS.

does anyone know of any?

I have no set of rules when prompting LLMs for CSS, it does seem to work more or less for me though.

What are your current issues or what limitations have you ran into?

mainly with trying to impart a particular art style.