What Gareth is saying, is that the essential/style/font-awesome.min.css should be renamed to fontawesome.css
Then you need to change the essential/config.php file to this...
$THEME->sheets[] = 'font-awesome.min';
$THEME->sheets[] = 'fontawesome';
so that it looks like this...
https://github.com/gjb2048/moodle-theme_essential/blob/master/config.php#L42
Hope this explains better for you?
Cheers
Mary