When you are using the Divi theme's default Theme Customizer Header (not using the Theme Builder header), you can notice a thin line showing just below the header area.
The thin line is a shadow effect under the header area.
If you want to remove the shadow effect with the default header and scroll header, you can add this CSS in WordPress Dashboard > Divi > Theme Options > Custom CSS:
#main-header, #main-header.et-fixed-header {
-webkit-box-shadow: none !important;
-moz-box-shadow: none !important;
box-shadow: none !important;
}
The above CSS will remove the Shadow for both the default header and the fixed header on scroll.
If you want to remove the shadow only for the default header, but not on scroll then you can use this CSS instead:
#main-header:not(.et-fixed-header) {
-webkit-box-shadow: none !important;
-moz-box-shadow: none !important;
box-shadow: none !important;
}
If you want to remove the shadow only for the scrolling header, but keep for the default header then you can use this CSS instead:
#main-header.et-fixed-header {
-webkit-box-shadow: none !important;
-moz-box-shadow: none !important;
box-shadow: none !important;
}
Save the Theme Options with your preferred code above and the shadow will be removed according to your option.