This one is pretty new, but I’ve found it to be the best CSS course I’ve ever taken:
If you’re a total beginner, fCC is the best place to start, but this course covers a lot of things that build on the basic things you can learn here. It explains a lot of really neat techniques for centering, building custom grids, and just generally writing more maintainable code. The word “advanced” in the title nearly scared me off - I’d say if you have a pretty strong understanding of the basics you can learn a lot from this.
I’d go on the record as suggesting any of Andrew Mead’s courses. I’ve taken a few of them, and he’s a phenomenal teacher. A lot of Udemy courses will just teach you to use stuff, but after going through about half of his React course, I actually feel like I “get it.”