[Twenty Twelve] custom color around only header and navigation area?

My site is under development and located at this link. I’m using Twenty Twelve with a child theme.

I’m not a big fan of how the current header/navigation area appears so I was thinking it would be cool to fill in the white area around the header and navmenu so it all “blends together” in the same color (darkblue). Here’s a photoshopped version of what I’m referring to (I made the entire header/menu area blue)

https://postimage.org/image/za2y7izs7/

I’m trying to wrap my head around how this would be possible to create on my site… possibly getting rid of the current white space around the header and enlarging the header image to stretch to the edges? Or maybe the CSS calls on this specific area and I can simply change it from the default white to the dark blue I’m after?

Any suggestions/ideas?

Thanks a lot.