You can setup the background color for the entire website from the Customizer > General Styling section.
Change Website Background Color
The site background color can be changed in a few simple steps. All you need to do is to click on the “Select Color” button in the Site Background Color area. It will expand with a couple of options to choose your desired color.
Once you chose the color, you will see a live preview with the background you selected presented on the website preview frame.