Control buttons (those on right of screens and 'more') are currently text only and as defined in the xul files. Add ability for facilitators to provide symbols for the control buttons. It's OK for the symbols to apply to all buttons with the same name, whichever the screen they are on.

Add a media folder /ControlSymbols which can contain suitable symbols; More, play, pause, next, previous, louder, quieter etc.
A better solution will allow control label to vary - perhaps using name-label format