The following will add/remove a class onto #navToggle so you can add an active class/change icons to an opened mobile menu - see example http://www.sanpedrocoffee.com.au/
<script type="text/javascript">
$(function() {
$('#navToggle').click(function() {
$(this).toggleClass("clicked");
});
});
</script>
#navToggle.clicked{
background:rgba(0,0,0,.5);
}
#navToggle.clicked:before {
content: "\f078";
}