I'm using the ASP menu control, .NET 4.0. I finally got the styles working thanks to the answer on this post but now that I've set the controlRenderingCompatibilityVersion in my web.config to "4.0" I'm seeing my separator image underneath the Menu control.
<div class="menuContainer">
<asp:Menu ID="_menu" runat="server" EnableTheming="true" DataSourceID="_siteMap" CssClass="menuClass" IncludeStyleBlock="false"
Orientation="Horizontal" StaticBottomSeparatorImageUrl="../images/menuSepImage.jpg" OnMenuItemDataBound="_menu_OnDataBound">
</asp:Menu>
<asp:SiteMapDataSource ID="_siteMap" runat="server" ShowStartingNode="false" />
</div>
I tried pulling out the default CSS styles that are auto-generated, and setting IncludeStyleBlock="false" but setting the #_menu img.horizontal-separator {} class in my CSS doesn't do anything, which is why I'm trying to set it with the StaticBottomSeparatorImageUrl property.