There are no tutorial for Modifying Power Store 3's design and it has been replaced by Power Store 4. There are tutorials for creating a custom Theme in Power Store 4 on the learning resources page.
Power Store 3 uses Dreamweaver templates for the Design.
Fro Details on working with DW Templates see this tutorial:
http://www.webassist.com/community/tutorials/view_tutorial.php?tid=140


