There seems to be a misunderstanding here

When you assign menu items to different themes , then joomla will render that menu item with that theme.
Basically if its a k2 item then joomla will simply render that item to be viewable by Storebox template view or style.
If you are trying to show Storebox homepage look on a menu item then you need to do following.
1. Create all the modules that is visible on Storebox Home look, from Storebox quickstart package and make them to show on your menu item "About PR".
2. Hide main content by using css code.
So you create a new set of modules using Storebox module positions same as it is in Storebox quickstart package.
I hope this makes it clear ; )
See you around...