Look in your theme.skin file in the skin folder
<portal:mojoTreeView runat="server" SkinID="SiteMenu"
ContainerCssClass=""
RootUlCssClass="art-hmenu"
RenderLiCssClasses="true"
RenderAnchorCss="true"
LiCssClass=""
LiRootExpandableCssClass=""
LiRootNonExpandableCssClass=""
LiNonRootExpnadableCssClass=""
LiSelectedCssClass="active"
LiChildSelectedCssClass="active"
LiParentSelectedCssClass=""
AnchorCssClass=""
AnchorSelectedCssClass="active"
ExpandDepth="-1"
ShowExpandCollapse="false"
PopulateNodesFromClient="false"
SuppressCornerDivs="true"
MaxDataBindDepth="-1"
SuppressImages="true"
/>
<portal:mojoMenu runat="server" SkinID="SiteMenu"
RenderContainerCssClass="true"
RenderImages="false"
UseMenuTooltipForCustomCss="true"
RenderCustomClassOnLi="true"
RenderCustomClassOnAnchor="false"
RenderLiSelectedCss="false"
RenderAnchorSelectedCss="true"
UlCssClass="art-hmenu"
LiCssClassWithChildren=""
LiCssClassWithoutChildren=""
LiSelectedCssClassWithChildren=""
LiSelectedCssClassWithoutChildren=""
LiChildSelectedCssClass=""
LiParentSelectedCssClass=""
AnchorCssClass=""
AnchorSelectedCssClassWithChildren="active"
AnchorSelectedCssClassWithoutChildren="active"
AnchorChildSelectedCssClass=""
AnchorParentSelectedCssClass=""
/>
Set the bold properties as shown.
Hope that helps,
Joe