Header Background Color (full width)

Hello!

I would like to change the background color on my header. And, I would like it to go across the entire width of the window. I mocked it up here:
http://www.sketchbookcloset.com/screen.png

Right now my site looks like this:
http://www.sketchbookcloset.com

(Obviously I will have to create a new graphic that is the inverse of what I have now. And, I’m going to remove the navigation below the title logo.)

So anyway – I’m getting hung up on how to revise the header.php & style.css to make this how I’d like it. I did a little searching around and I was able to change the “Masthead” color – but it was confined to an area and it didn’t span the entire width of the window. I feel like this is probably a stupid solution that I’m just not seeing.

(Also, I am not using a child theme because I did not do that… out of impatience/confusion and now I feel like it’s too late to go back & try to start over.)

I appreciate any help with this!

Thank you!