This is the first time I’ve been asked this, so I’m not sure the exact way to properly do this. I may change my answer if I come up with a better way
…..But I guess you could make a background for the #head-wrap section since it is directly behind the #header and then make the #header as wide as possible. The thing is, I think you have to specify a size for the header in the function file. Maybe you could just leave it at 1152 since thats the width of the theme, and have a background image for the #head-wrap be full width.
Again, this could be completely wrong, I’ve never been asked this, so I haven’t really put much thought into it before.