asp.net - Ajax ModalPopupExtender 样式问题

Question

我遇到的问题是我的 Css 已被我的控件使用。这是我的代码

ASP.NET

  <cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="LoginBtn"
        CancelControlID="btnClose" BackgroundCssClass="modalBackground">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center"Style="display: none">
        This is an ASP.Net AJAX ModalPopupExtender Example
        <br />
        <asp:Button ID="btnClose" runat="server" Text="Close" />
    </asp:Panel>

CSS

.modalBackground {
background-color: Black;
filter: alpha(opacity=90);
opacity: 0.8;
}

.modalPopup {
background-color: #FFFFFF;
border-width: 3px;
border-style: solid;
border-color: black;
padding-top: 10px;
padding-left: 10px;
width: 300px;
height: 140px;
}

我尝试了很多事情，例如从示例中复制粘贴，但我无法正确解决我做错了什么？