You just have to modify the styles listed in the menu control declaration:
Code:
<asp:Menu ID="aspnetMenu" runat="server" Orientation="Horizontal" StaticTopSeparatorImageUrl="images/tab-sep.gif"
StaticEnableDefaultPopOutImage="False" StaticDisplayLevels="2" MaximumDynamicDisplayLevels="<%$ Tokens:AppConfigUSInt, MaxMenuLevel %>" >
<StaticSelectedStyle CssClass="aspnetMenu_StaticSelectedStyle" />
<LevelMenuItemStyles>
<asp:MenuItemStyle CssClass="aspnetMenu_Level1" Font-Underline="False" />
<asp:MenuItemStyle CssClass="aspnetMenu_Level2" Font-Underline="False" />
</LevelMenuItemStyles>
<StaticMenuItemStyle CssClass="aspnetMenu_StaticMenuItemStyle" />
<DynamicSelectedStyle CssClass="aspnetMenu_DynamicSelectedStyle" />
<DynamicMenuItemStyle CssClass="aspnetMenu_DynamicMenuItemStyle" />
<DynamicMenuStyle CssClass="aspnetMenu_DynamicMenuStyle" />