ASP.NET / GridView：HeaderStyle-CssClass没有效果

<asp:GridView ID="gvTable" runat="server" AllowSorting="true" ShowHeader="true">
  <Columns>
    <asp:BoundField DataField="ActivityDate" HeaderText="Date"
      HeaderStyle-CssClass="date" />
    <asp:BoundField DataField="ActivityType" HeaderText="Type" />
    <asp:BoundField DataField="ActivityNotes" HeaderText="Notes" />
  </Columns>
  <PagerSettings Position="Bottom" Mode="NextPrevious" PageButtonCount="5"
    PreviousPageText="Older activities" NextPageText="Newer activities" />
</asp:GridView>

<thead>

"date"

ItemStyle-CssClass

<thead>
  <tr>
    <th scope="col"> <!-- No class :( -->
      Date
    </th>
    ...
  </tr>
</thead>