Work-around to add custom icons to Domoticz when the gui for icons isn't working for MacOS or Linux clients, and you don't have acces to a windows client.
Without any support or guarantees.
The plugin uses the python plugin frameworks methods to add any custom icons. Icon zip files must meet the Domoticz custom icon specs. Just put he icon zip files in the plugin directory (like: /home/pi/domoticz/plugins/iconic), (re)start the iconic hardware and the import will start. Let it run once and disable the plugin (hardware) immediately after the import has finished. Remove the zip files.