may I ask why ?
From this page
php-scripts-and-solutions/
Each pre-built PHP solution provides you with all the required source files, images, and database settings so you can easily customize any feature you want. 
and
PowerStore A complete pre-built online store with content management, starting point themes, and a complete administrative backend.
I am not trying to be difficult, though god knows some of the questions I ask can be obtuse, I just want to understand why custom themes are not supported
