Thanks for hopping in the forum. This forum is really geared towards supporting the forum, so it might be best to reach out to MailChimp Support, for details on styling the embed code. Support can be contacted on this page: http://mailchimp.com/contact/
With that in mind, all pages in the signup process, such as the confirmation pages, can be customized in your MailChimp list. These guides from the MailChimp knowledge base have details on customizing those pages: http://kb.mailchimp.com/article/how-do-i-build-and-design-my-signup-form-and-response-emails
Let us know if you have any other questions.