custom css option?

Thanks for this plugin! I’m interested in the custom css option. Is this only available using the shortcode? I’m using it as a widget and I’m hoping I could make my own css file to customize the look of this form. Any ideas or options I might be missing?

I have “Show Widget Style” set to “no” but I’m not sure where to add the external css.

Thanks,
Scott

http://ww.wp.xz.cn/extend/plugins/vertical-response-newsletter-widget/