I think this is all you need:
#MainNav, body #megaMenu ul.megaMenu > li.menu-item > a, body #megaMenu ul.megaMenu > li.menu-item > span.um-anchoremulator, body .megaMenuToggle, body.home #megaMenu ul.megaMenu > li.menu-item.current-menu-item > a {
border-bottom-color: #d00;
border-bottom-width: 2px;
}
#megaMenu ul.megaMenu > li.menu-item > a, #megaMenu ul.megaMenu > li.menu-item > span.um-anchoremulator, .megaMenuToggle {
margin-bottom: -2px;
}
Be sure to set the border-bottom-color
to your value in the first style. Also, from the “Appearance > Theme Options” area set the background color of the menu to the same as the page (white is default) and the layout style to full width. That should be all you need.