In EXTRA.css template add:
/* FA icon for Overlay Close button */
.xenOverlay a.close:after {
content: "\f00d";
font-family: FontAwesome;
font-size: 14pt;
padding-left: 12px;
}
.xenOverlay a.close {
background-image: none !important;
}
/******/
Font Awesome needs to be called via CDN...