.modal_window, #modalMask /*.ui-widget-overlay*/
{
    display: none; /* don't show it */ /* styling bellow */
}

/*.ui-widget-overlay*/
#modalMask
{
    position: absolute;
	overflow: hidden;
	top: 0;
	left: 0;
	background-color: #000;
	z-index: 9998;
}
.modal_window
{
    z-index: 9999;
    position: absolute;
}