Any CSS file on your site that loads on that page should be fine. You can add it to your template's CSS file, for instance. Alternatively, you can add it to the /components/com_jfbconnect/assets/jfbconnect.css file. However, that change will be overwritten when you upgrade JFBConnect, so that may not be the best spot for it.
I hope that helps, but let us know if you need more.
Thanks,
Alex