Reduce header width to match body

Hi and maybe someone can help me.

I’m working on https://www.peca-ni.org using the excellent Hueman theme.
I have a child theme installed and enabled.

I’ve put a background image set on the site (because the Hueman theme is a quite dark), but the header is full width while the site is manually set to 1160px.

My question:
How can I reduce the header width to match 1160px, so the background image is visible at the sides of the header?

Thank you for any help provided
Yaddler