Are you planning on going into web or software development? If web I'd suggest getting to know HTML quite well as you'll have to use it a lot, even if you don't do front end. Git is also very good to know, as is SQL.
Positioning is a big part of CSS. But CSS positioning with
display: inline-block; etc can be difficult and hacky at times, and some back end devs dislike the entire language for that reason. Flexbox is a relatively new spec that fixes a lot of those issues. If you don't want to spend too much time on CSS I'd probably opt for learning flexbox positioning. It will be faster to learn and you're less likely to end up hating CSS, plus the browser support is fairly good in 2017.