Usually you improve such skills if you have to build and maintain things :)
the important thing is to do both -> why ? because you have to fix your own mess and the more you do it the clearer structure and concepts get.
If you just build things you probably wont get a good grasp on the downside of the picked approach
if you only maintain things you can always blame the others because they made the "stupid decisions"
and ofc reading up on atomic css, functional css, media queries inside of classes or outside containing different classes, explicit element behaviour vs composed element behaviours and so on helps to stay up to date.
and in time you're actually able to explain why you picked certain approaches and if you can do that you basically are "good" most of us remain in the "okay" or "don't know we actually suck" level :) like I do ... but i'm a backend guy ....