You can override any of the artisteer css by putting the override css lower in the css (ie lower in the style.config file will make it come in lower than css files higher in the list). This is the "Cascading" part of Cascading Style Sheets, css rules closer to the element take precedence, hence rules lower in the css take precedence over things higher in the css and inline styles on the elements take precedence over css.
However this might be problematic if you do want the titles styled by artisteer elsewhere. In the next release of mojoPortal we will have a way to add a custom css class to the body element on any cms page and this will make it possible to override things on specific pages without affecting other pages.
Best,
Joe