The license under which the CumulusClips software is released is the GPLv2 (or later) from the Free Software Foundation. A copy of the license is included with every copy of CumulusClips, 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 CumulusClips code inherit the GPL license.
We consider all plugins, language packs, and themes to be derivative work and thus inherit the GPL license. If you disagree, you might want to consider another video sharing platform.