the answer depends on your skin (and to a lesser degree on your portal setup). In many cases skins will contain all the style definitions within their skin.css so you make the amendments there e.g. perhaps a skin is installed in your primary portal and has a name of "dweet" so you would find it at portals/0/skins/dbsweet/skln.css .In other cases the skin developer has added the markup within the page so you would need to edit the html (if the skin came as a html skin) or the resultant ascx.

I recommend the skinning page on the wiki http://www.dotnetnuke.com/Resources/W... to get an idea of how skinning is done which should allow you to reverse engineer your skin. It's also a good idea to use the built in developer tools (press F12 in IE or Chrome or install firebug if using firefox) to select an element and see how it get's it's styles e.g. heres an article showing IE's developer tool http://msdn.microsoft.com/en-us/libra...