I got it to work, but not sure it's the best method.
I added the font-family: "FontAwesome";
I also added <link rel="stylesheet" href="
https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css"> to page_container <head>
I thought that FontAwesome was included with XF 2.x? It didn't seem to work unless I added the stylesheet to page_container.