100% Width Header & Nav Question

Hello!

I'm Dani, a designer working on customizing the Lifestyle Pro theme for A. M. Leibowitz (site is technically live, but not finished and has no incoming links since it's still in development). I've also attached here what I envisioned the site looking like with the header.

Full disclosure: I'm much more of a front-end developer, though I am ever so slightly conversational in PHP. In looking at the HTML output, it looks like I would just want to break the header block out of the site-container div and make it 100% (with varying breakpoints on the header image for tablet and mobile), but I'm not sure exactly how to go about doing that. The header.php file for Genesis contains no HTML, and I'm just...not sure where to go to make the header and navigation work the way I had it laid out in the mockup. Anyone have any ideas or able to point me in the right direction to be able to make this work out? Thanks so much!