Without knowing what OS you have on your machine I suggest you find a built in utility called “Character Map” or like.
Go to the “Private Use Area” in the map list to find icons. There you can view and search for icon like characters and see what U+xYYY code to put as content (If you look in the character description, it’s the UTF-16 number you want)
As @PaulOB already said, you set the icon you want by using the class name for it in the html. That is how it is set up now in the site you linked to.
As I mentioned above, the site is using the “Ico-Moon” font imported from the “template.css”: /2015/helix3/media/mod_languages/css/template.css and the ruleblock you posted is in that file.
If you want to use the “Awesome” font instead you have to change font family and unicode to match the icon you want.