Using separate css for included show_news.php?

I have a basic website layout and I've managed to get the show_news.php to display inside one of the divs My problem might just be that I'm not really familiar with php (yet), but here's what happens:

The show_news.php gets the css formatting from the css for the actual page and not the default.css (under /skins). How can I define the show_news.php to use the skins/default.css -file? If I'd use iframes it would work out of the box, but I guess the problem is that the show_news.php has been implemented on the page/inside the div like this:

<?php include ("cutenews/show_news.php"); ?>

Am I completely lost here or what?