What are some accessibility concerns that come up in CSS?

Hidden content is a big one here. It’s only acceptable to use display: none; if you’re both trying to hide things visually and the content itself.
CSS controls colors so color accessibility is relevant. Focus styles are also important and directly controlled by CSS.
There is a lot more accessibility stuff that is related to HTML and JavaScript, so mentioning that stuff is great, but I think it’s interesting to target the question solely on CSS to force focused thinking.