Solved Horizontal line in Header 5

In mobile, it will revert back to use the default Divi header, so that you can have slide in / fullscreen menu in mobile: https://divi.help/threads/for-mobile-menu-how-to-show-slide-in-or-fullscreen-menu.646/

So to change the background color of the mobile header, you just need to change it in Theme Customzier > Header & Navigation > Primary Menu Bar > Background Color

As to get rid of the line, try the below CSS in Divi > Theme Options > Custom CSS:

Code:

#main-header {
    box-shadow: none!important;
}