How do you change the design of the Mailchimp widget?

I have installed your Mailchimp plugin and have it working perfectly. My problem is that I cannot control the design of the signup form at all in the footer area of the client's site.

First name / Last name appear the same size.
Email appears smaller and looks horribly out of place.

All I want is the form fields to look the same.

What am I doing wrong? I have installed it on other sites with no issues at all and no other customization required.