Change GridView column's ItemStyle and AlternatingItemStyle

I have a GridView:

<asp:GridView ID="gridResults" runat="server" AllowSorting="True" OnSorting="gridResults_Sorting" >
<Columns>
<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="1" />
<asp:BoundField DataField="Category" HeaderText="Category" SortExpression="2" />
</Columns>
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