Thanks for the comparison shots there. In looking at the code for the site, it appears there's some CSS that's overriding the styling for the button on the other pages. Specifically, the background color is getting changed to "white", which makes the button look... strange. After I manually changed the styling in the CSS, things looked okay.
Basically, what you'll want to do is add some styling for the MailChimp button classes:
When I was testing in Chrome, I set it to blue and got the following results:
Hopefully that helps a bit with getting things straightened out!