Icon sets Fidelio supports user icons and custom button icons. However, the graphics need to be available to fidelio in the form of icon sets. These icon sets are available on the web and from the Window's client for Hotline. You can specify multiple icon sets, and they will be searched in the order given for icons and sounds.
First match is used.
Note that icon sets also contain the sounds that fidelio plays when sound support is enabled, so you will need at least one icon set to hear any sound. Transfer this file to Linux and register it in the preferences, so that fidelio, when restarted, will use it. Bad Moon Icons has many common user icons. Use only in conjunction with a more updated version, as some icons are missing.