Teisia,
From what I have seen, you are probably going to have to create custom styling with CSS for the specific form design you choose. I have not found anything like what you are looking for, but I have been playing for a while with creating custom CSS for the forms and I noticed that depending on the form style, and the buttons you select, the final code rendered by Marketo is different to style the forms.
I might be mistaken, but that is what I detected a few weeks ago. I am planning on creating my own guide based on what I find as I do more and more customizations. If I get some time to create an actual document that could help others, I will probably post it here in the community site.
So far the only documentation I have found does not include that.
http://developers.marketo.com/documentation/websites/forms-2-0/
Good luck!
Rafael