Hi,

Try looking in the skin CSS file. You should see some specifications on the border. If you don't want any borders at all, just set the widths to 0px.

Example:

.MainMenu_SubMenu {
 z-index: 1000;
 cursor: pointer;
 cursor: hand;
 background-color: #FFFFFF;
 filter:progid:DXImageTransform.Microsoft.Shadow(color='DimGray', Direction=135, Strength=3);
 border-bottom: #FFFFFF 0px solid;
 border-left: #FFFFFF 0px solid;
 border-top: #FFFFFF 0px solid;
 border-right: #FFFFFF 0px solid;
}