As you may have noticed, Divi adds under the main header a separator, a shaded line, that separates it from the main content.
I mean this:

Well, there will be situations (webs) in which you prefer not to show that separator line, and it is very easy to achieve.
You just have to add this bit of CSS:
/* Hide header separator Divi */
#main-header {
-webkit-box-shadow: none;
-moz-box-shadow: none;
box-shadow: none;
}
Code language: CSS (css)
To add the code open the administration section Appearance → Customize → Additional CSS, paste the code and publish the changes.
The result looks like this:
