Newbie trying to have background with 2 colors (one matching header/footer)

Warning…newbie here…!

I am trying to modify my background so that the top and the bottom of my background are the same color as my the header and footer, basically like https://www.cnet.com (where the background on the right and left sides of the page at the top and bottom are black, matching the header and footer, but the background in between the header and footer is grey).

I’m getting confused if the top and bottom of the background (outside the width of the content area) is set via the background color (or an image), or are those areas driven by the header and footer?

I am doing this based on the Whitespace theme, which I have been slowly modifying as I learn.
Thanks much!
J