The Divi Theme's options screen (found at "Divi > Theme Options") presents a long list of settings which can be used to configure the theme. At the top of the options screen is the header, which includes tabs for accessing the different collections of settings ("General", "Navigation", "Builder", etc). Normally this header scrolls off the top of the screen when you scroll down. If you want it to stay visible, instead, you can use the following CSS:
This is CSS for the admin area, so instead of adding it via a child theme or the custom CSS box, you will need to add it using a hook such as "admin_head", or a plugin such as Add Admin CSS.
I find this CSS useful when I want to create a screenshot showing the location of a setting which appears near the end of the options list. With the header visible, I can highlight the full path to the feature in one go, e.g.