rocknbil - 3:49 pm on Sep 2, 2011 (gmt 0)

If it's an external web site, no. Imagine if you could - you could change all the links in an iframed page to suit your needs.

If it's on the same domain, there are a variety of ways you could do it - pass a parameter to the containing page to make it use a different style sheet, for example.