WordPress themes must be GPL
WordPress themes must be licensed under the GPL says WordPress lead developer Matt Mullenweg. A question over whether or not themes created for the WordPress blogging system have to be licensed as GPL version 2 or can be released under a more restrictive license had emerged within the WordPress community and Mullenweg has now responded with a definitive statement.
To reach his conclusion, Mullenweg consulted the Software Freedom Law Center and asked for their opinion. They concluded that because of the intermingling of PHP code and HTML, including WordPress API calls, program logic and loops, that almost any theme created for WordPress would be a derivative work of the GPL licensed WordPress default themes. The SFLC said that it could be possible to create a non-GPL theme for WordPress but doing so would lose most, if not all, WordPress functionality.
Graphics and CSS files fall outside the derivative work definition and could be under a seperate licence though. In light of this, Mullenweg says that WordPress.org will only promote and host themes that are "100% GPL or compatible".