Theme skin Font-Size won't work for GridView CommandField Buttons

[Burt]

I am trying to move all control attributes to a theme skin file but cannot
get GridView Button accept a Font-Size attribute. Everything else works fine.
<asp:GridView ....
<asp:CommandField ButtonType="Button">
<ControlStyle Font-Size="X-Small" />
</asp:CommandField>
</asp:GridView>

I have even tried setting a default Button attribute but it won't work for
CommandField buttons in a GridView.
<asp:Button runat="server" Font-Size="X-Small" />

There must be a way to do this. Any suggestions. Maybe it a bug.