A bit old of a post, but I remember I learned CSS specifically to code on a site like this by just looking through W3School's CSS course. Of course not the WHOLE thing, but learn about what you would need.
For example, if I wanted to change something about my profile's section titles, then I'd just focus on finding the selector I should use and then searching up how to change the property of that. CSS is very similar to HTML, since it's very repetitive. For CSS, it's just a bunch of...
selector { property: value; }
over and over again. It also helps to look at other layouts' code for inspiration, or to see how they did something specifically. As for making layouts, I usually just use Inspect Element to preview my code.
This was a bit choppy, but hope this helps! :-)