Unfortunately, Artisteer does not (yet) officially support mojoPortal, so the style-artisteer-overrides.css file is used to bridge the gap between what Artisteer exports as an HTML template, and what mojoPortal requires to be styled above and beyond that. One example are the WYSIWYG editors in mojoPortal. Special CSS needs to be provided for those, because they are not styled by Artisteer. Without the additional styling, special effects like headings, bullets, etc. will not appear in the editor display.

Generally speaking, when you're working with Artisteer, it's best to make any changes to colors, fonts, widths, etc. in the program, and then rebuild your skin according to the Creating Skins with Artisteer documentation. That way you will have an updated Artisteer .artx file that you can continue to build on in the future. For things that you just can't do with Artisteer, put them into the style-artisteer-overrides.css file, or create your own custom css file and add it to style.config for your skin.

Hope that helps,
Jamie