2012 page background elimination?

I am using InstantWordPress on my computer (to learn). So, no site link yet.

I have created a site. I am using the 2012 Theme. I have created a child Style.css file which seems to work fine.

I want to eliminate the page back ground (if that is the correct term). I have already changed the background with an image (which is outside of the page).

I would like to know the coding to use in my child Style.css file to accomplish the task. https://www.fmaatc.org illustrates the look I would like to create (BTW – https://www.fmaatc.org is not a WordPress site).