Yeah, JS… I guess you need to apply the general background to the whole page like I did and then the sidebar background needs applied with JS to measure the distance between the sidebar and the left side to calculate % and adjust the css.
Ok, so I think I found a solution here. I’m using body:after to create a “faux background” that starts from the left of the browser window. The benefit of this is no extra divs, and just a few lines of CSS. I then give my content div the same background as the body and voila!