Slide In - How can I center the MailChimp email box?

Working on a client site ( Have the Slide-in Plugin installed and have found how to customize colors, etc. However, I can't figure out how to center the MailChimp linked email box. If you go to the site, you'll see what I mean (and screenshot attached).

What code do I need to center that email box?

Thanks in advance for your help.