What you now want to do is not special for MediaWiki, you just want to know how to use CSS. I think you will find good tutorials, when you just feed Google.
CSS offers settings for margins, for paddings, for background-color, for (text) color, for font-size, font-family, and many, many, many more. You can basically change the complete styling of your site with it - and with complete I actually mean "every aspect".
E.g. you can get white text for these headlines like so:
h2 {
background: url("skins/common/images/catbg.jpg") repeat-x scroll 0 0 #990000;
color: white;
}
You should try out Firebug, an extension for Firefox. It allows you to modify CSS styles of a page while you view it in the browser. That way you can quickly get the results you want. Then copy your changes in Commons.css and you have it. :-)