Josh WC is cool, but if you want to (re)learn modern CSS and how to leverage it properly, from first principles, take a close look at "Axiomatic CSS" as described and demonstrated by
a profoundly compelling, persuasive, expert, pragmatic, accessible, maintainable approach to CSS. It's transformative. I've been doing webdev since the late 90's (ie, before CSS), and this is hands-down the best such resource I've ever encountered. The axioms and some key examples are free; paying $100 for the book and component generator and access to updates was beyond a bargain. No affiliation, just a happy customer.