def _set_fallback(elements, src_field, fallback, dest_field=None):
        """
        Helper function used to set the fallback attributes of an element when
        they are defined by the configuration as "None" or "-".
        """

        if dest_field is None:
            dest_field = src_field
        if isinstance(fallback, six.string_types):
            fallback = elements[fallback]

        attrs = elements[src_field]
        elements[dest_field] = (
            attrs[0] if attrs[0] is not None else fallback[0],
            attrs[1] if attrs[1] is not None else fallback[1],
            attrs[2] if attrs[2] is not None else fallback[2])

        Helper function used to set the fallback attributes of an element when
        they are defined by the configuration as "None" or "-".
        
rtv/theme.py