Stuck in Twenty Ten theme

I have been working on a new site using WP3.0 and the Twenty Ten theme for now. I just installed 2 new themes and then I try to preview them, I still get the Twenty Ten appearance in the preview window. The other themes are very different in appearance. (Arras Theme 1.4.3.1, and Magazine Basic 2.6.8)

I did make some changes to the style.css file in the Twenty Ten theme and added some plug-ins but I can’t see how this should matter. I didn’t try actually activating the new themes yet as I think this might really mess things up.

Any ideas on what could be happening? Specific File Permissions I should check?