@classmethod
def get_css_classes(cls, instance):
    """
        adds all content from css_class_fields to css_classes.
        """
    css_classes = super().get_css_classes(instance)
    if instance.glossary.get('extra_css', None):
        css_classes.append('c-extra-%s' % instance.id)
    for k in getattr(cls, 'css_class_fields', []):
        xc = instance.glossary.get(k)
        if isinstance(xc, str):
            css_classes.append(xc)
        elif isinstance(xc, list):
            css_classes.extend(xc)
    return css_classes
