Include skeleton-fontawesome-buttons.css in your HTML:
skeleton-fontawesome-buttons.css
<link rel="stylesheet" href="skeleton-fontawesome-buttons.css">
Add the .button-icon class to your buttons:
.button-icon
<a class="button button-icon"> <i class="fa fa-cog"></i> </a>
Table cells are smaller than buttons, so there's a special case for using the .button-icon class in table cells.