Good question there. You can use those. The icons that are used in the theme are made with a different set that’s bundled with the theme. To go with the Genericons, you’ll need to load that font for use on the front end of your site. There’s a plugin at https://wordpress.org/plugins/genericond/ that can do that for you.
From there you can then replace one of the built in styles with one of the genericons. Take the star one as an example to replace. You’ll need to edit the style.css file (you can do that by going to the Appearance section in your dashboard and choosing edit). The Custom CSS in the theme options will filter out this type of style for security reasons.
Here’s what to add at the bottom of the style.css file: