This seems to be more insidious than just CSS leakage from your theme. It looks like there is some script running that is hunting down all submit buttons and imposing a class of .btn on them so it is this that you are seeing. Perhaps you could try adding to your WP theme CSS file something along the lines of…
#spMainContainer .spButton.btn { …replacement styles… }
obviously replacing the styles that the class .btn defines with your own. Not sure of that would work as, of course, I am unable to try it but it should be a starting point.