Theme, skin and css for gridview button field

I have a website that I would like to apply css to all of the command button fields for all of the gridviews.
In my default.skin I have:
<asp:GridView runat="server" CssClass="mGrid"> <columns> <asp:ButtonField> <ControlStyle CssClass="button" /> </asp:ButtonField> </columns> </asp:GridView> When I execute the page the buttons appear as small blocks with no text and the rest of the gridview does not render.
When I comment out the <columns> and <buttonfield> here in the .skin file and set the buttonfield contolstyle-CssClass="button" the buttons and gridview render properly.
So I know that the .css for 'button' works okay when called for a specific control.
What am I doing wrong with the .skin file? 
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