code: def get_field_label_css_class(self, bound_field):
          
          class_name = self.field_label_css_class
  
          if bound_field.errors and self.field_label_invalid_css_class:
              class_name = join_css_class(
                  class_name, self.field_label_invalid_css_class)
  
          return class_name or None
docs: Returns the optional label CSS class to use when rendering a field template.
  
          By default, returns the Form class property `field_label_css_class`. If the
          field has errors and the Form class property `field_label_invalid_css_class`
          is defined, its value is appended to the CSS class.
  
          :param bound_field: `BoundField` instance to return CSS class for.
          :return: A CSS class string or `None`