Actually, I don’t see any border below the Header itself…
Can you attach some screenshot?
If you think on the red gradient background below the navigation menu items,
probably it should be configured from the MEGA MENU settings (not the part of the theme )
But, if for some reason you can’t find the option to remove it, you can insert the next code into Customizing->General->Additional CSS:
@media only screen and (min-width: 601px){
#mega-menu-wrap-agama_nav_primary #mega-menu-agama_nav_primary > li.mega-menu-item > a.mega-menu-link{
background:none!important;
}
}