For this theme we used FontAwesome 5, the most recent version that, for us, is the most complete set of vector icons and social logos you could ever find. Font Awesome Free is still open source and completely free for commercial use.
Of course, you can now get a PRO license which offers you everyhing you need from such a library.
To make use of these icons, simply copy the icon’s label and paste inside the
<i class="fas fa-heart"></i> <i class="fas fa-heart fa-2x"></i> <i class="fas fa-heart fa-3x"></i> <i class="fas fa-heart fa-4x"></i>