I use CSS on a regular basis and forget the parts I don’t use often. So, even if you do one project, unless you continue using CSS Daily, you may find that you forget the some of the specifics.

I’ve found these CSS games very fun and useful for zero effort, pleasurable, highly effective and well structured learning:

https://flukeout.github.io/

https://cssgridgarden.com/

https://flexboxfroggy.com/

Some resources I find myself using all the time:

Https://css-tricks.com

https://developer.mozilla.org/en-US/docs/Web/CSS

Https://caniuse.com