To change your email's background color, you'll want to navigate to the "Global Styles" section on the top of the email controls. A color selection modal will pop up; here, you will select the desired color from the section titled "Background Colors."
Select your desired color and watch the background change.
Another helpful tool is the "Apply all changes to all modules." This action will save your settings as you work from module to module. If you ever want to reset a specific module back to its original state, select the "Reset to Global Styles Value" icon. This selection is visible when you hover near the setting you wish to reset.