In the css folder there is a user.css.
I expected that every css in sj-basic4.css can be overruled in the user.css. But it does n't work.
For example, if I want to change the backgroundcolour of header-top, I copy the css from sj-basic4.css, paste it in user.css and change the code. See screenshot.
So what am I doing wrong?

you added more CSS code change the background color of header-top in the folder templates/sj_basic4/css/sj-basic4.css add code:
.header .header-top {background: #01687F;}

If I change this not in user.css but in sj-basic4.css, I expect the changes get lost when the template will be updated?

I think it's not affected.
Kind Regards