One of the difficulties with Drupal is that with so many modules needed to make a good site you can end up with a dozen or more different CSS files. On occasions when you need to tweak the CSS to match a specific design, or fix something for IE6, it can take quite a while to dig down to the exact definition you need. You might try throwing random snippets of CSS at the problem to try to make it go away, but they usually won't work.

What I've found is that to successfully override CSS you have to repeat the exact same definition as the original, no matter how obtuse it my seem. So if you want to override one simple paragraph you may have to assign some pretty strange CSS, eg.:

* html .span-6 {margin-right: 5px;}

If that's what is already being used, then that's what you have to do to override it.

It may be a bit frustrating, but following that simple guideline has saved my bacon several times.