Outside of my extreme envy of the SEO they are going to get out of this, Rachel is spot on here. Learning CSS has some pillars, like language syntax, selectors, layout, and flow that, once learned, unlock your CSS merit badge.
What I would add is that if you really want to learn CSS, give yourself a project that has real-world stakes (like a personal website), so what you are doing with CSS feels important and gives you the incentive to do interesting things and get it right.
Keith Grant has some interesting thoughts on relating the concept of a "Common Core" to CSS:
For example, if you need to add 32 + 67, break the problem up to 30 + 60 and 2 + 7, both of which are much easier to do in your head. As someone who excelled at math in school, I have found that most of these Common Core tricks are things I discovered on my own as a student, and are precisely why I was able to do well in the subject.
We need common core tricks like this for CSS.