ehowey

You want to look for the following line in the style.css file:

#menu-primary li ul {

Change the background: #ccc; to whatever color you want it to be.

Mine is a bit odd because I have customized the theme so heavily but something like the following:

#menu-primary li ul {
display: none;
position: absolute;
top: 25px;
left: 0;
border-bottom: 1px solid #fff;
border-top: none;
border-left: none;
border-right: none;
background: #000;
z-index: 100;
}