To begin customizing your website’s Layout & Typography, you must first enter the Theme Customizer by clicking on the Divi > Theme Customizer link within your WordPress Dashboard. This will launch the customizer. Next look for the “General Settings” panel. Click the panel to open up the relevant settings and begin customizing. Within the General Settings panel, you will notice three sections: Layout Settings, Typography and Background.
Originally posted on Divi’s site https://www.elegantthemes.com/documentation/divi/customizer-layout/