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)