I think what dsaro is referring to is the mailchimp css that is auto-included when the signup form. The one that shows up when I open up Chrome developer tools or Firebug is http://www.example.com/?mcsf_action=main_css&ver=3.3.2
So for example, if I was trying to change .mc_signup_submit to have the property of text-align: left in style.css, the mailchimp CSS would overwrite that rule with its default text-align: center
