Basically, wordpress themes are free or paid. Who doesn’t like free stuff? Sadly, there isn’t anything REALLY free. I researched over 30 sources of free wordpress themes (50+ themes), and these sources are top ranked for the term “free wordpress theme”, didn’t find one theme REALLY free.

All of them have hide JavaScript codes. There are several types of the codes:

• The theme contains several footer links. And these links can’t be easily removed.
• The theme contains hidden links on homepage. Usually 3-5 links. And it’s not visible.
• It contains hard-coded codes which hijacks your Adsense codes or affiliate ID. These kind of themes are normally ranked for “Adsense theme” or “affiliate theme”. These themes are evil. They are really nice looking themes. Once you install them, they steal the money from you. Some of them use sneaky codes. For example, they don’t hijack your theme 100% of the times. They took a percentage, like 50%. So you can still earn money from you project and won’t think you might be hijacked. In this case, you can still find the problem if you open your affiliate link and look at the PID carefully. There’s another kind of code which only hijack your site when people open your site from Google search results. This means that if you open your site directly as an administrator. You don’t find it.
• Malicious codes contained. Normally it’s under function.php. You site traffic will be directed. In cause case, most of the search engines will mark your site as unsafe. Chrome and firefox even block you site quickly.Malicious codes contained. Normally it’s under “function.php”. You site traffic will be directed. In cause case, most of the search engines will mark your site as unsafe. Chrome and Firefox even block you site quickly.

  So be careful of your theme. If you don’t have a trustful theme designer, use wordpress default theme. Not perfect looking, but you are safe.