Bi-Color Background Colors

I absolutely love this theme! I saw another blog which seemed to have figured out how to create a bi-color background so that the area on either side of the header is a different color than the rest of the background. Can you guide me in how to accomplish that?

https://www.frugalwoods.com/

Notice the dark gray color near the header vs the much lighter gray on either side of the content. Thank yo!