We found a solution last year. I can't remember what fixed it. Here is what I have in my stylesheet. I looked in the skin and did not find anything. If it does not work, here is the site if you want to look at the code to see the rest of it.
.SubMenu /* outher left and right bg of unselected sub item, bg of sub tab */
{
filter:progid
XImageTransform.Microsoft.Shadow(color='red', Direction=135, Strength=0);
z-index: 1000;
cursor: pointer;
cursor: hand;
font-family: Verdana, Tahoma, Arial, Helvetica;
font-size: 11px;
font-weight: bold;
color: #FFF;
height: 16px;
background-color: #CD3300;
background-image: none;
background-repeat: no-repeat;
border-left: #98AAB1 0px solid;
border-top: #98AAB1 0px solid;
border-right: #98AAB1 0px solid;
border-bottom: #D9562B 1px solid;
}