Arastta is released under the GNU GPLv3 license from the Free Software Foundation. A copy of the license is also included with every copy of Arastta, but you can also read the text of the license here. Part of this license outlines requirements for derivative works, such as extensions or themes.
May I user another license for non-PHP files?
May I sell extensions or themes?
Yes. However, you must distribute all of the files under the GNU GPLv3 license.
May I restrict extension/theme installation per domain/period?
No. It's againts GPL license.
May I restrict extension/theme support per domain/period?
Yes. The GPL license is applicable only to the software.