custom skin base.css


I am currently creating custom skin for our portal, the visual editor that comes with wm Portal only let’s me change basic elements, but i need to customize more specific elements that are defined in the base.css. I have been able to modify these specific elements creating a custom css for my project and overwriting classes from the base css, but I would like to modify the look and feel in the base.css.
Is there a proper way to access all the elements in the base.css?