The entire site

From the dashboard, click on Divi >> Theme Options then scroll to the bottom of the page to “Custom CSS”. Insert something like this, where #bfd4eb is a color code. You can also use color codes like “lightgray” or “blue”.

#main-content {
    background-color: #bfd4eb !important;
}

Sections, Rows, and Modules

Display the front page, click on “Enable Visual Builder” at the top, and navigate to the section/row/module you want to change. Click on the “Settings” gear, then click on “Background”. Change the background color to the desired color code or color name.