向模式弹出窗口添加样式

I have added a modal popup AJAX control to my website.

When a button is clicked it correctly appears but im running into trouble with the styling.

At the moment I have added a new style to my site master Site.css style sheet but it is not taking effect on the popup.

I have a separate style sheet called 'modalBackGround.css'

This is the css class:

body 
{
background-color:#000000; 
}

This is my modelpop AJAX control, note the ccs property:

        <cc1:ModalPopupExtender ID="Button2_ModalPopupExtender" runat="server" 
        TargetControlID="Button2"
        BackgroundCssClass="modalBackGround" 
        DropShadow="true" 
        OkControlID="btnOk" 
        CancelControlID="btnClose" 
        PopupControlID="Panel1"  >
    </cc1:ModalPopupExtender>

How do I correctly apply this style sheet to the background of the popup to solve the transparency issue?

Sorted this. Please note. To anyone who is basing their pages on a site master. You can declare multiple css style sheets through the use of the 'link' property within the 'head'. This will then allow you to use these styles on every page as it is declared on the master.