Java Script Popup box displays in front of a white blank web form(.aspx)

I am displaying a Java Script popup message box from my C# code. It works
good, but the problem is that when the message box displays, the entire background
color of the web page turns white? how can I prevent this? I just want the popup
to be displayed in front of the existing web page.

// Here is my C# code
  if (!ClientScript.IsStartupScriptRegistered("Information"))
     {
      Page.ClientScript.RegisterStartupScript(typeof(Page), "Information", "<script type=\"text/javascript\">Popup()</script>");
     } 

// Java Script code in my .aspx page
function Popup()
 {
   var confirmMessage = " You have selected Project (80+ hours). Upon submit, this ";
   confirmMessage = confirmMessage + "\n action will direct you to the Add Project for ...