set .ui-widget-overlay with jquery

HTML

<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css">
<script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
<div id="dialogDiv">dialog div</div>
<input id="btn" type="button" value="click me!!!">

CSS

.ui-widget-overlay{background:grey;}
#dialogDiv{width: 1000px;}

JavaScript

$("#dialogDiv").dialog({autoOpen:false,modal:true});
$("#btn").click(function()
                {
                  $("#dialogDiv").dialog('open');  
                });