window.addEvent('resize', function() {
    var viewport = document.getSize();
    if($chk($('bg'))){
        $('bg').setStyles({'width': viewport.x, 'height': viewport.y});
    }
})

window.addEvent('domready', function(){
	var viewport = window.getSize();
	$('bg').setStyles({'width': viewport.x, 'height': viewport.y});
})