<style>
.divclass{
  font-size:5em;
  color:grey;
  cursor:pointer;
}
 
.divclass:hover .fa-thumbs-o-up:before{
  content:"f164";
  color:green;
  opacity:0.6;
}
</style>

<div class="divclass">
  <i class="fa fa-thumbs-o-up"></i>
</div>