[Plugin: MailChimp Widget] Change button color

Hi there,
I am using the plug-in, but I want to change the button color (black on black doesn’t work so well) Were do I need to do that? I see no image, no style.css and I can’t locate it in my theme’s style.css

Hope you can help me, thanks!

http://wordpress.org/extend/plugins/mailchimp-widget/