Asp.net 应用css背景时，模式弹出框消失

我正在使用Ajax模式弹出扩展程序将面板显示为模式弹出。我已经将CSS类应用于面板，所有内容都显示良好。但是，当我将

BackgroundCssClass

属性添加到弹出扩展器时，面板上的边框不会显示。模式背景显示正确，面板中的所有其他内容保持不变，但我无法显示边框。这可能是什么原因造成的

**CSS**

.PopupPanel
{             
width: 75%;
height: 55%;
text-align: center;    
padding: 5px;
overflow: auto;
border-style: solid;
border-width: 8px;
border-color: #0055A5;    
background-color: White;    
}

.ModalBackground
{
background-color: rgba(170,170,174,0.5);    
} 

<asp:Panel ID="InputPanel" runat="server" CssClass="PopupPanel">

        <asp:Label ID="HeaderLabel" runat="server" 
            Text="[text here]" 
            Font-Size="Large"></asp:Label>

        <br />            
        <asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine" Width="85%" 
            Height="65%" Wrap="False"></asp:TextBox>                        
        <br />


        <asp:Button ID="okButton" runat="server" Text="Submit" 
            CssClass="genericButton" />
        <asp:Button ID="CancelButton" runat="server" Text="Cancel" 
            CssClass="genericButton" />
</asp:Panel>

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
    PopupControlID="InputPanel" TargetControlID="PasteButton" 
    BackgroundCssClass="ModalBackground" PopupDragHandleControlID="HeaderLabel">
</ajaxToolkit:ModalPopupExtender>

**ASPX**

.PopupPanel
{             
width: 75%;
height: 55%;
text-align: center;    
padding: 5px;
overflow: auto;
border-style: solid;
border-width: 8px;
border-color: #0055A5;    
background-color: White;    
}

.ModalBackground
{
background-color: rgba(170,170,174,0.5);    
} 

<asp:Panel ID="InputPanel" runat="server" CssClass="PopupPanel">

        <asp:Label ID="HeaderLabel" runat="server" 
            Text="[text here]" 
            Font-Size="Large"></asp:Label>

        <br />            
        <asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine" Width="85%" 
            Height="65%" Wrap="False"></asp:TextBox>                        
        <br />


        <asp:Button ID="okButton" runat="server" Text="Submit" 
            CssClass="genericButton" />
        <asp:Button ID="CancelButton" runat="server" Text="Cancel" 
            CssClass="genericButton" />
</asp:Panel>

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
    PopupControlID="InputPanel" TargetControlID="PasteButton" 
    BackgroundCssClass="ModalBackground" PopupDragHandleControlID="HeaderLabel">
</ajaxToolkit:ModalPopupExtender>