The best way to learn CSS is to be introduced, then immediately apply the concepts at hand in a real world project. That's exactly what we'll be doing here—no learning individual CSS properties without actually seeing how they should be put to use. As a result, we need some sort of project to work off of.
Web projects are typically developed off of some sort of design mockup. Some of the top design programs today are Figma and XD, while some sites are still developed in programs such as Sketch, Photoshop, and even Illustrator.
To ensure you're up-to-speed with modern times, we're going to be developing our site off of a Figma design I created:
Designs are typically handed off from a designer to a developer, but don't let that deter you from trying your hand at design. Developers can be designers, and don't ever let anyone tell you otherwise! I designed a huge chunk of the chriscourses.com website even though my main background is development. It is possible to design, even as a dev, it's just a skill you have to put time into (like anything else), to get good at.
If you'd like to learn more about design, I can't recommend the book Refactoring UI enough—it's how I learned my main design skillset, and how I was able to put together the same design we'll be using in the Figma link above.