http://mathoverflow.net/content/flair-Default.StackOverflow.css
Since the contents of an iframe is a separate XML document, that CSS file overrides any previous page-specific CSS. To override that one again you have to include more CSS into the contents, can you do that?
I may be wrong on exactly how CSS and iframes interact. As frames aren't valid XHTML, I don't use them so I'm not totally sure. However this question backs me up and also tells you how to do it with javascript if you can't do anything properly.
I used firebug to look at the page in question since firebug tells me what CSS rules are being used to render a particular element and where those rules come from.
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