Remember Laurence Olivier asking Dustin Hoffman; “is it safe,” in the old movie Marathon Man?
The same question can be asked of your WordPress theme; “is it safe?” If you are uncertain as to the legitimacy of a theme, its developer and source – particularly if it’s “free,” precautions are available. You can have the theme checked prior to uploading it coupled with some potentially nasty malware or such.
Head over to the site Theme Check where you can upload the theme and have it scanned free of charge. Theme Check scans for security issues and code quality and provides a detailed report and score when finished. If you do not have the theme file to upload you can search the database by the theme name. The scan criteria is based from the WordPress official Theme Review Requirements and Theme Unit Test Data discussed here at WordPress.org.
Theme Check also has a free plugin available for use in scanning your theme file if already installed to your site.