Hi Matt,
The article you mentioned is intended for Forms 1.0, our old editor - customers who started with Marketo this year don't have access to it because we're trying to slowly phase it out of use.
This article walks through how to customize the form button style and color in Forms 2.0:
https://community.marketo.com/MarketoArticle?id=kA050000000LH9CCAW
You might also consider using some of the resources we have here to further customize it:
http://developers.marketo.com/documentation/websites/forms-2-0/
like example #5, which explains how to make the form submit due to a click event on an element that's not part of the form, and then hide the form button using CSS (it has the class "mktoButton").