How often I use
I probably use CSS several days out of the week, since I am the most experienced developer on the team in terms of front-end work.
My skills with CSS
Like HTML, I can write quite a lot of CSS without even looking up a property. I have some experience with CSS grid and flexbox, and can write pseudo-properties. I love that, for small projects, you can now use CSS variables in vanilla CSS. I can't make fancy illustrations or animations in CSS, but I can do simpler animations and use transitions.
I try to be very conscious of things like color contrast and focus state, as well, because not only are these important to make a site accessible, but they make sites easier to use for all people.
Frameworks and pre-processors
I almost always use some form of pre-processor; I have the most experience with LESS, but can also find my way around SASS. I like the way that you can nest and structure the CSS to match how your code is structured. I have used functions in both of these as well as made use of the native handling of variables.
I am very comfortable and experienced with the Bootstrap framework, and can write code in versions 3 and 4 largely without having to look at the specs. I have also used Semantic UI, which has some nice controls, but is not very accessible.