From GroupScript documentation
How do I change color of '...'?
If you want to change something in the appearance of the site (like background color, font size) you should use a plugin for FireFox called Firebug - it has object inspector, that allows you to point & click on specific element, and see which CSS file and even which lines apply to particular element.
Please be aware that you need a solid understanding of CSS to do changes of such kind.
Refer to http://www.evotech.net/blog/2007/06/introduction-to-firebug/ or http://docs.joomla.org/Tutorial:Using_Firebug_With_Your_Joomla_Website to introduction and unofficial user guides for Firebug.