The license under which the Build software is released is the GNU from the Free Software Foundation. A copy of the license is included with every copy of Build software, but you can also read the text of the license here.

Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of Build software code inherit the GNU license.

There is some legal grey area regarding what is considered a derivative work, but we feel strongly that add-ons plans and themes are derivative work and thus inherit the GNU license. If you disagree, you might want to consider a non-GNU platform.