[Theme: ComicPress] Pagewide header background image

I am using the ComicPress theme and wants to extend the background image of the header and footer so it covers the width of the page, witohut changeing the width of the top menu and body content.

See https://www.creaturebox.com (massive centered image in header – and black footer) and https://pvponline.com/ (yellow and black header – and black footer) perhaps it becomes a little more clear what I am after.

I have tried to set the background-operations: center in the styles.css file but without success.

Does anyone have any sugeestions on how to make this work? Many thanks in advance,

Jonny

https://www.ads-software.com/extend/themes/comicpress/