Power Store is not(yet) a template driven ecommerce script.
Maybe WA designers can put this top of "our wish list" ;-)
I personally had a lot of problems with customizing the left menu on PS3. Finally I dropped it and used a second CSS Menu on the left.
Globally PS3 has been styled by 3 main CSS files, common,home and content.
Common CSS controls the global look and feel of the entire site. Home CSS belongs to main(index) page of the PS3. With common CSS you can control the content in the editable area of the default PS3 template.
After a lot of hours (headache and coffie ;-) ) we were able to create a different PS3 template. You can see the result on www.magicware.nl