I would like to assign a special icon to each of the buttons in the fep_menu. I tried assigning a new class to the buttons using the ‘fep_menu_buttons’ filter, just to realise that fep-button and fep-button-active classes where rather important, so I tried defining an ID to the buttons using the same filer, however, it doesn’t work, probably because ID is not defined in the parent array.
Do you have any suggestions on how I can achieve to style each button differently?
You can pass different class for different button. Next version you will be able to pass id also.
In your code you are overriding full button, it will be better if you override just what you need. eg. use $menu['newmessage']['class'] = 'fep-button your_class';