There's lots of courses out there similar to FreeCodeCamp, etc. but I think you'll find, like I did, that design is a different beast. It's not something that is codified into rules that are easy to reason about. It's fluid, it's a feeling more than a rule to follow. And what's more? It changes all the time. What was considered "trendy design" in 2005 is now old and antiquated (although, the same could be said of the JS ecosystem, so maybe it's not all that different after all)

That said, there are some "rules of thumb" that can be learned and applied to your existing projects.

Some of my favorite resources for learning design as an engineer:

https://refactoringui.com/ - The book is slated for release soon but I've followed the authors on Twitter for a while now and Steve in particular has some great advice for "designing" as an engineer [1].

https://dribbble.com/ - One thing I've found as an engineer is that most people recognize good design when they see it even if they can't create it. I've spent a lot of time picking out good designs from good designers and replicating them on my own. As I do so I try to reason about why they made that design choice versus something else. Over time I find myself thinking things like, "OK, so you gave the card a larger border radius because you wanted it to feel softer." It's just something you pick up with practice and deliberate effort.

[1] https://twitter.com/i/moments/880688233641848832