If you want to add your own icons, we recommend using icomoon app. There, you can generate your icons (upload or choose from the list of icons).
Then after generating fonts, you can download them. You’ll get a folder with the files that you can replace the default ones in the css/icons folder.
Be careful to keep the structure of the original icons folder the same. In the icons folder there should be: demo.html, icons.css, icons.php, licence.txt, selection.json and icon font files.
The icons.php is an array of all existing icons. You can use programs like Sublime Text and Notepad++ to easily copy all the icons from the icons.css file here, preserving the same structure as in the original file.
Note that the icomoon will give you files that have different names (style.css for instance). Be sure to rename it all to have a working icon set.
You can also use icons provided by The Creator if you wish (there are 8 different font files with plethora of icons to choose from).