How do I change the background colors for my theme?

I am using the Bamboo-10 theme. My site-in-progress is at https://www.the-gardenschool.org/test/.

Basically, what I’m trying to do is change the background color on the outermost part of the page (the wide left and right margins that are gray). I can’t find the code that will let me make that change; have looked everywhere through style.css and other files.

I’d definitely appreciate any help anyone can offer. Thanks!