Need to add .wrap div

Hello Wes,
The legacy theme css has the site-inner div with a width of 1200px, and no .wrap div within it.

Because of the client’s design, I need to change that so the site-inner is 100% width, and add a .wrap div for the 1200px width. Specifically, I need to add a background image to the .site-inner div that will be full browser width.

I can’t figure out where/what file I need to modify to add that .wrap div within the site-inner div.
Can you point me in the right direction?

Thank you!
Danna