Themes not showing correctly on newly transferred blog

I recently transferred my domain name (and blog) to another server/hosting site. The original theme was not showing correctly, and I thought it was a problem with the theme, so I decided to buy themes. However, after uploading the themes, I realize they were not showing correctly either. I went on a few forums (just read them) and apparently it has something to do with the style sheet. Found out about W3C Validation and validated 2 out of three themes, and the results were errors and warnings. I have no idea how to fix them and no idea what to do. Can anybody help me?