def _init_custom_background_appearances(self):
background_args = {}<EOL>for key in self._background.known_states:<EOL><INDENT>background_args.update({<EOL>k[len('<STR_LIT>'):]: v<EOL>for cls in self.__class__.__mro__<EOL>for k, v in cls.__dict__.items()<EOL>if k.startswith(f'<STR_LIT>')<EOL>})<EOL><DEDENT>if background_args:<EOL><INDENT>self.set_background(**background_args)<EOL><DEDENT>
Setup the background widgets from parameters found in dynamically named
`custom_...` class variables.

For example, the class variable `custom_base_image = ...` would call 
`set_appearance(image=...)` on the "base" background widget.