Been too many years so I no longer remember many of the sites where I learned about CSS. Liz Castro's Pigs, Gourds, and Wikis site comes to mind though it's been quite a while and she has moved on. The W3C's
starting with HTML + CSS, most of the
W3schools site perhaps starting with their
CSS Tutorial.
Open epubs and study the CSS to see how various creators handle their CSS. I prefer the keep it simple CSS styling when it can be done.