This will make the navigation "sticky"and overlapping the hero (slider) with NO BACKGROUND (transparent) when the page loads, and when you scroll down, the background will appear.

Go to divi >> theme options >> custom css and add:
.et_fixed_nav.et_show_nav #page-container, .et_non_fixed_nav.et_transparent_nav.et_show_nav #page-container{
padding-top:0 !important;
}

Before



After