Thanks for writing in! In order to troubleshoot the issue a bit further, I'll need a bit more information. Do you think you could send me a screenshot at mailchimptk[at]gmail.com, showing the CSS changes on your WP Dashboard that you're referring to?
Also, when you say you updated to the newest version, are you specifically referring to upgrading the MailChimp List Subscribe Form plugin, or did you upgrade your WordPress version? And finally, can you clarify a bit about the email template you're referring to? The plugin should be separate from your templates and campaigns in your MailChimp account, so it shouldn't necessarily carry over interms of the style or design or your plugin sign up form.
We look forward to hearing back!