How would I add overlay to the background behind my dialog box?

How would I add overlay to the background behind my dialog box?

This is my first time coding anything. I am trying to add a 75% opacity to the background behind a dialog box. I have no idea where to add code, let alone what code to add. I'm adding it to a magento website (if that matters) and my code works fine the way it is, just no overlay.  I am very new here so please be gentle. Thanks

This is the code I'm using. 

<!doctype html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>jQuery UI Dialog - Modal confirmation</title>

  <link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css">

  <script src="http://code.jquery.com/jquery-1.9.1.js"></script>

  <script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>

  <link rel="stylesheet" href="/resources/demos/style.css">

  <script>

  $(function() {

    $( "#dialog-confirm" ).dialog({

      resizable: false,

      height:300,

width:500,

      modal: true,

      buttons: {

        "Enter": function() {

          $( this ).dialog( "close" );

        },

        Cancel: function() {

       window.location = "http://www.google.com";

.ui-dialog-content {

    opacity: 100;

}

 

 

        }

      }

    });

  });

  </script>

</head>

<body>

<div id="dialog-confirm" title="WARNING">

  <p><span class="ui-icon ui-icon-alert" style="float:left; margin:0 7px 20px 0;"> </span>The text of my dialog box goes here</p>

<img src="http://img.photobucket.com/albums/v427/4012948/silhouette_zps11fb0eb7.jpg?t=1389315744" alt="Placeholder Image" />

 

</div>

 

<p> <p>

 

 

</body>