By default YUI3 assigns yui3-widget-mask class for background shading. To override that we need to change its style properties. Similarly for position of the panel, it can be changed by specifying position property of .yui3-panel class. These are the methods to accomplish this:‘.yui3-widget-mask’).setStyle(‘background’,’#fff’);‘.yui3-panel’).setStyle(‘position’,’fixed’); 
Full code is like this:
YUI().use(‘panel’, function(Y) {
EX_waitPanel = new Y.Panel( {
srcNode : ‘#plswait’,
width : “240px”,
centered : true,
close : false,
draggable : false,
modal : true,
bodyContent : waitImg,
headerContent : waitMsg