Hi,
Try these steps. Go to settings > MailChimp setup in your dashboard and uncheck the “Custom styling: Turned on?” checkbox. Next paste the below code in the “Custom CSS” section of the theme options.
#footer #mc_signup_form .mc_input {
background: #686868;
border: 1px solid #444;
}
Hope this helps. If you run into any issues send me the url with a temp login to your installation and I will fix it.