cannot override marketplace theme css

I’m having trouble over-riding the css rules that are part of MarketPress. By this I mean the modern.css, icon.css, and classic.css. These are the rules that create the buttons and so forth.

I’m trying to modify them by adding rules to my child theme but it doesn’t work. Probably because the plugin styles are loaded after the the theme styles. I keep having to you the !important tag (and even that doesn’t seem to work in some cases). Is there a work around for this?