Icons - Talon
Here you can find all the theme bundled icons.
If you want more icons, all you need to do is go to Customize > Icons and add a CDN link to the icon pack you want to load. A popular examples is here.
Please note: To use the new icons you've loaded manually, please refer to their documentation. For example, if you load FontAwesome: instead of adding in the formaticon-mobile, you'll be adding in the format fa fa-flag.