This is the exact code from that item:
<h3><span class="fa-stack"><i style="color:#1E72BD;" class="fa fa-circle fa-stack-2x"></i><i style="color:#FFF" class="fa fa-flash fa-stack-1x"></i></span> Lightweight & Fast</h3>
I’m using Font Awesome’s stack option, where you can stack two icons over each other.
So I’m putting the circle icon behind the lightning (flash) icon.
There’s lots of Font Awesome examples on this page: http://fortawesome.github.io/Font-Awesome/examples/