A friend called me and informed me that he had found two links in his footer to spam websites. He "found" the premium theme, which he was using "on the Internet". I have looked at the theme again and the footer.php was consisted of the following (shortened by me):
I have decoded the footer and it was "only" the original footer with two additional links to spam sites, together with the associated keywords. It could have been also some harmful code that makes it possible to break into your blog for example. What exactly the code is, that is not seen for a person without profound coding expertise.
Imagine that it is possible to break into the blog, or potentially damage the entire web space of the server. I wouldn't like to pay the invoice of your provider for the damage and surely don't want to lose my whole website and files. And all this because you wanted to save $59!
If you want to have a special theme, buy it! The theme developer also provide support within the $59, also many developer help with customization, updates and much more. A pretty good deal in my opinion.
So take the advice and rather buy a premium theme than download it for "free" somewhere else. The same applies also for premium Plugins.