Wow that's great Nick, CSS can be a ton of fun! The Mozilla Developer Network includes a lot of reference articles about CSS properties, but they also contain some great how-to articles that explain concepts and things that are otherwise hard to google for.
Learn CSS: developer.mozilla.org/en-US/docs/Learn/CSS
Example of a useful article I link people to regularly, How the Stacking Context works: developer.mozilla.org/en-US/docs/Web/CSS/CSS_Posi…
There's also a new section on MDN called the CSS Layout Cookbook, which includes recipes for common layout tasks, and if there's more than one way to do it, it explains the tradeoffs so you understand what you're working with: developer.mozilla.org/en-US/docs/Web/CSS/Layout_c…
Another great resource for learning CSS and staying up to date with CSS is css-tricks.com but keep in mind that this is a blog-site site, not a wiki, so you can still read and find really old and outdated information on there - just be aware of the publishing date. The new content on this site is some of the best CSS content around.
Lastly, one that has helped me a lot is CSS-Tricks sister site, Codepen. I found that I could get better at CSS by practicing things, but rather than trying to build a whole website, I could make a Codepen and experiment with 10 button styles, or a handful of text shadow styles, or just work on improving my understanding of one little skill in isolation at a time. There are already thousands of pens on there you can look at and learn from too, and it's a nice community: codepen.io