Modifying what HTML MarketPress outputs

My apologies if this is dealt with somewhere and I just haven't found it, I did my best.

I'd like to know if it's possible to edit the various MarketPress pages. I have MarketPress installed and I`m using my own custom Wordpress theme. I'm talking about for instance: the category, product and checkout pages.

I have read and understand:

wpmu.org/the-easy-guide-to-theming-marketpress/

and

/wp-content/plugins/marketpress/marketpress-includes/themes/Themeing_MarketPress.txt

But. Is it possible to just modify the existing pages, the default template? Without creating a whole new MarketPress template? I would just like to do things like adding classes to the existing buttons, etc...

Thank you, sincerely,

Michel