1. Check the source code for the page, and click on the URL to the .css stylesheet. If it returns a 404 page then you know that the CSS link is broken.
2. Try reuploading all of your files. Usually this fixes a lot of problems. I would normally say just ignore the css problems on this page, but the Pligg Admin Panel uses the same CSS file, so if its missing or corrupt you will want to fix it.