Theme Enigma: implement custom font icons

Hi there, I’m using a customized instance of the enigma theme. In the service-section it shows some icons that you can choose from font-awsome and implement by a shortcode. But as they don’t provide the icons I would like form my servicetopics, I need to customize the theme. I already tried to load my cutom-icon-font and replace the font-tag in the css but it does not work. The fontawsome-icons disappear but my own icons will not show. Has anybody a descirption for me (tutorial) how to ge along?
Thanks! Isa