how to put "Processing" dialog to the middle of the screen?

<?taglib uri="http://www.zkoss.org/dsp/web/core" prefix="c"?>  
<window>
<style>  
.z-loading {  
	font-weight: bold;  
	padding:2px; 
	position:absolute;
	left:200px;
	top:200px;
	
}  
.z-loading-indicator {  
	color: gray;  
	border:0 none; 
	position:absolute;
	left:200px;
	top:200px; 
}  
.z-loading-icon{
	position: relative;	
}
</style> 
	<button label="Test Progress"
		onClick="org.zkoss.lang.Threads.sleep(2000)" />

	<listbox id="list" width="300px"></listbox>
</window>