Styling django non-field errors on forms

If I look in Django's forms.py, as_p() calls _html_output(), which styles field errors with self.error_class() (although I can't locate the definition of that).

However, _html_output() does NOT style non_field_errors (a.k.a. top_errors in the code).

How does one style the non-field errors? Cut and paste all of _html_output?

I am using Django 1.0.