Modeldelpopup CSS Class

 .modalBackground
        {
            background-color: White;
            filter: alpha(opacity=40);
            opacity: 0.5;
        }
        .ModalWindow
        {
            border: solid 1px White;
            background: Gray;
            padding: 0px 10px 10px 10px;
            position: absolute;
            top: 180px;
            left: 22px;
            height: 206px;
            width: 439px;
        }

 <cc1:ModalPopupExtender ID="MPE1" runat="server" BackgroundCssClass="modalBackground"
                            TargetControlID="LnkBranches" PopupControlID="Panel1" DropShadow="true">
                        </cc1:ModalPopupExtender>