There are two places where the background color can be specified:
1) At the FULL WIDTH ELEMENT
Click into an object and using the breadcrumb navigation, click on FULL WIDTH ELEMENT (FULL WIDTH ELE...)
This presents you with the option to change the background color on the Content Tab.
By Default, Background color is off (See white color with a red slash below).
2) Within an object.
If you click into any object, you'll see the Defaults tab next to the Content tab. Here you can also specify the Background color for the template.
You could have both background colors displayed at the same time:
Note: If you specify at the FULL WIDTH ELEMENT and the Object Defaults, it's possible that the FULL WIDTH ELEMENT will prevent the Object Default color from displaying.
If so, please remove the FULL WIDTH ELEMENT color by clicking on an object, navigating the breadcrumb back to the FULL WIDTH ELEMENT, and picking no color.