Cześć Marek,

use more class names taken from body

body.site.helix-ultimate.predefined-header2 header#sp-header { }

then for the browser it should be more important.

and also use background instead background-color

use those two ideas together. Editing template.css file doesn't make sense at all.

If helped, mark topic as solved. If not, I have alternative idea.

I was wondering why the custom CSS wasn't working, but I changed it in theme.scss. I know it's risky. I might forget, but I hope I'll remember when I need to.