Sub Menu Background Border Radius

I've been trying to find the CSS code to give a radius to the background of the sub-menu but have not been able to. I used this code to change the border radius of the nav-menu. Also, there's a funny thing happening - when you hover over a menu item on the main nav-menu and then move away from that item, the border radius momentarily disappears exposing a perfect rectangle. Any idea why this might be happening?

.main-navigation .main-nav ul li:hover > a, .main-navigation .main-nav ul li:focus > a, .main-navigation .main-nav ul li.sfHover > a {
border-radius: 20px;
	margin-bottom: 5px;
}