Themes in asp.net

Creating themes / skin file in a folder App_Themes ,Extension for themes /Skin is : .skin

<asp:GridView SkinID="GridViewSkin" runat="server" CssClass="gridview" >
        <AlternatingRowStyle CssClass="grdalt" />
        <PagerStyle CssClass="grdpgr" />
</asp:GridView>

<asp:Calendar width="250" DayStyle-Width="34" DayStyle-Height="15"   runat="server"
    CellPadding="0" CellSpacing="0" FirstDayOfWeek="Monday" ShowGridLines="True"
    DayStyle-BackColor="#F99B98"  DayStyle-ForeColor="Black" DayStyle-Font-Names="Arial"
    NextPrevFormat="ShortMonth" SkinID="CalendarSkin"
    SelectionMode="None">
    <DayHeaderStyle BackColor="Black" Font-Names="Arial" ForeColor="White" Font-Size="9pt" />  
    <DayStyle Font-Names="Arial" ForeColor="Black" Font-Size="9pt">
    </DayStyle>
    <TitleStyle BackColor="Gray" Font-Names="Arial" ForeColor="White" HorizontalAlign="Center" />
</asp:Calendar>

<asp:TextBox SkinID="textboxSkin" CssClass="styleNormal" Width="180px"></asp:TextBox>