Ajax control toolkit modalpopup black background issue

Hi
       I am learning using ajax control toolkit and I am trying to use Ajax control toolkit modal popup extender in one of my website. Modal popup is working fine however I am not able to get black background when modal popup is opened.

Here is my code:

<asp:LinkButton ID="LinkButton11" runat="server">Open Pop up</asp:LinkButton>

        <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="PnlPopup" TargetControlID="LinkButton11" CancelControlID="LinkButton12" />
        <asp:Panel ID="PnlPopup" runat="server" CssClass="message">
            <h3>It Popped Up Here is modal popup</h3>

            <asp:LinkButton ID="LinkButton12" runat="server">Close</asp:LinkButton>
        </asp:Panel>