The key to have a transparent menu is to apply transparency on the menu group's Style property. All the built-in menus have solid background color style. So they are not transparent. In order to make them transparent, you must remove the background color style attributes and add transparency attributes. For example, sub menu for the following menu is 50% transparent:
Code: CSS
.sub_menu
{
filter:alpha(opacity=50);
opacity: 0.5;
-moz-opacity:0.5;
font-size: 9pt;
font-family: Verdana;
color: black;
cursor: hand;
border: #999999 1px solid;
padding: 2px;
}
Code: HTML/ASPX
<eo:Menu runat="server" ID="Menu1" ControlSkinID="None" Width="360px">
<LookItems>
<eo:MenuItem
HoverStyle-CssText="background-color: #cccccc; border-right: #999999 1px solid; padding-right: 4px; border-top: #999999 1px solid; padding-top: 1px; border-left: #999999 1px solid; padding-left: 4px; border-bottom: #999999 1px solid; padding-bottom: 1px; "
ItemID="_Default" NormalStyle-CssText="background-color: transparent; border-right-style: none; padding-right: 5px; padding-left: 5px; padding-bottom: 2px; padding-top: 2px; border-top-style: none; border-left-style: none; border-bottom-style: none; color: black"
SelectedStyle-CssText="BORDER-RIGHT: #999999 1px solid; PADDING-RIGHT: 4px; BORDER-TOP: #999999 1px solid; PADDING-LEFT: 4px; PADDING-BOTTOM: 1px; BORDER-LEFT: #999999 1px solid; PADDING-TOP: 1px; BORDER-BOTTOM: #999999 1px solid; BACKGROUND-COLOR: white; ">
<SubMenu ItemSpacing="3" LeftIconCellWidth="12"
Style-CssClass="sub_menu" ShadowDepth="0">
</SubMenu>
</eo:MenuItem>
</LookItems>
<TopGroup>
<Items>
<eo:MenuItem Text-Html="Download">
<SubMenu>
<Items>
<eo:MenuItem Text-Html="Developer Downloads">
</eo:MenuItem>
<eo:MenuItem Text-Html="Subscriber Downloads">
</eo:MenuItem>
<eo:MenuItem Text-Html="Microsoft Download Center">
</eo:MenuItem>
</Items>
</SubMenu>
</eo:MenuItem>
</Items>
</TopGroup>
</eo:Menu>
Transparency is specified in CSS rule "sub_menu". This rule is then used by Menu -> LookItems -> _Default -> SubMenu -> Style-CssClass.
In order to use transparency, you will also need to set the sub menu's ShadowDepth to 0.
Thanks!