CSS is an amazing tool, and in the last few years it has grown a lot, introducing many absolutely fantastic features, like CSS Grid, Flexbox and CSS Variables.
Before you check out those cool features, take a step back and inspect the CSS basics through my introduction to CSS post.
Here are some more advanced topics regarding the modern layout construction in CSS:
Sometimes there are things you think you know, but when it comes to implementing, they are a bit confusing, especially if you start searching online, where you can really find anything:
Variables is a huge topic, one that could make CSS be very close to SCSS and the other preprocessors:
CSS is huge on displays, but it's also huge for printing documents. Check out how to use CSS for print if you're interested.
Web Fonts are another big topic. My CSS System Fonts article sheds some light on using system fonts and avoid loading unnecessary bits of data if you don't strictly need them.