Add the solpartmenu classes to your skin.css file.

here is an example from one of my skins that's pretty close to what you want, you'll just have to tweak the color settings where necessary to get your exact look and feel.

/* Main Menu */

.MainMenu_MenuContainer {
 border-bottom: #FFFFFF 0px solid;
 border-left: #FFFFFF 0px solid;
 border-top: #FFFFFF 0px solid;
 border-right: #FFFFFF 0px solid; 
 background-color: #BCBCBC;
}

.MainMenu_MenuBar {
 cursor: pointer;
 cursor: hand;
 height:16;
}

.MainMenu_MenuItem {
 cursor: pointer;
 cursor: hand;
 color: white;
 font-family: "Arial Black", Arial, Helvetica;
 font-size: 14px;
 font-weight: normal;
 font-style: normal;
 border-left: #FFFFFF 0px solid;
 border-bottom: #FFFFFF 0px solid;
 border-top: #FFFFFF 0px solid;
 border-right: #FFFFFF 0px solid;
}

.MainMenu_MenuIcon {
 cursor: pointer;
 cursor: hand;
 background-color: #BCBCBC;
 border-left: #000000 0px solid;
 border-right: #000000 0px solid;
 border-bottom: #000000 0px solid;
 border-top: #000000 0px solid;
 text-align: center;
 width: 15;
 height: 21;
}

.MainMenu_SubMenu {
 z-index: 1000;
 cursor: pointer;
 cursor: hand;
 background-color: #BCBCBC;
 filter:progid:DXImageTransform.Microsoft.Shadow(color='DimGray', Direction=135, Strength=3) ;
 border-bottom: #404040 0px solid;
 border-left: #FF8080 0px solid;
 border-top: #FF8080 0px solid;
 border-right: #404040 0px solid;
}

.MainMenu_MenuBreak {
 border-bottom: #404040 0px solid;
 border-left: #FF8080 0px solid;
 border-top: #FF8080 0px solid; 
 border-right: #404040 0px solid;
 background-color: #BCBCBC;
 height: 0px;
}

.MainMenu_MenuItemSel {
 background-color: #394E6C;
 cursor: pointer;
 cursor: hand;
 color: white;
 font-family: "Arial Black", Arial, Helvetica;
 font-size: 14px;
 font-weight: normal;
 font-style: normal;
 border-left: #000000 0px solid;
 border-right: #000000 0px solid;
 border-bottom: #000000 0px solid;
 border-top: #000000 0px solid;
}

.MainMenu_MenuArrow {
 font-family: webdings;
 font-size: 10pt;
 color: black;
 cursor: pointer;
 cursor: hand;
 border-left: #000000 0px solid;
 border-right: #000000 0px solid;
 border-bottom: #000000 0px solid;
 border-top: #000000 0px solid;
}

.MainMenu_RootMenuArrow {
 font-family: webdings;
 font-size: 10pt;
 color: black;
 cursor: pointer;
 cursor: hand;
}