Give this link a try – http://bit.ly/1toMF0I It explains how to add a custom css file which will cause it to have priority and override the default css.
There are plenty of other tutorials out there if you need more info – https://www.google.com/search?client=opera&q=add+custom+css+in+genesis&sourceid=opera&ie=UTF-8&oe=UTF-8 ….but that should do it.
