Is there a way to CSS-style a 3rd party iframe? For example, with a Twitter widget, I would like to change the font size.
How would I do this?
I don't think you do... For all intents and purposes that iframe is another document and is completely outside of your control (as long as it's 3rd party and / or stored in another domain).
In the case of twitter you can always configure the widget itself when you start it but as long as you're trying to access / alter most properties of the document inside the iframe, you're sold.
Long story short: as far as I know you're dependant on whatever tools the 3rd party provides you. Of course, you can always do hacks (intensive curl calls, parsing and rewriting) but that would be like killing a fly with a really big cannon.
What are you exactly trying to achieve?. Maybe there's a way the 3rd party allows you to...