docs: def apply_widget_css_class(self, field_name):
          """
          Applies CSS classes to widgets if available.
  
          The method uses the `get_widget_css_class` method to determine if the widget
          CSS class should be changed. If a CSS class is returned, it is appended to
          the current value of the class property of the widget instance.
  
          :param field_name: A field name of the form.
          """
          field = self.fields[field_name]
          class_name = self.get_widget_css_class(field_name, field)
  
          if class_name:
              field.widget.attrs['class'] = join_css_class(
                  field.widget.attrs.get('class', None), class_name)