reveal i is a class, so calling it with the class attribute selector.

IPython/nbconvert/templates/slides_reveal.tpl

@@ -59,7 +59,7 b' html {'
   border: 0px solid black;
   box-shadow: 0 0 10px rgba(0, 0, 0, 0);
 }
-reveal i {
+.reveal i {
   font-style: normal;
   font-family: FontAwesome;
   font-size: 2em;