Whatever course module is up next is most likely irrelevant to your current design goals. The further along you get in your passion project, the more things you will want to learn out of necessity. I’ve spent over four hours reading a boring technical documentation — to fix a single icon.
Whenever I build something in CSS, I first work on the layout with extremely simple styles. The goal is to get everything in the right place, not to make it look good. The first step is making sure the backend is outputting everything I need. I want to get that out of the way and then be able to spend all my time in the same CSS file. Everything gets a background: red or background: blue just to verify that the selectors are working properly. Then I work on the layout and positioning, thinking about things like responsiveness and edge cases. Once those are fully figured out, I usually make a new stylesheet to fill in all the design details and make things look nice.