Glad to hear you got things going.
I have not made ANY sites with powerstore, but have created the functionality of a store (small one) using other DW extensions (Power CMS, DataAssist, iWrite, SecurityAssist). If ever I were required to make a fully featured all-singing/dancing store I would use PowerStore.
To have a look at what other folks are doing with PowerStore (and other WA tools) have a browse through These
For the css I would first back-up the existing stylesheet(s) so that I can revert to this if I get in a mess, then start by opening the template and seeing what styles I want to apply site-wide, and use the styles panel to make changes. I would need to have a pretty good idea of the effect each change to the stylesheet was having throughout the site, so would be inclined to check pretty much all the pages in 'live' view - or - previewing in different browsers after each change to make sure I wasn't running into problems.
Changes to specific pages (assuming there were just one or two) I would do by adding a class to the element css, and keeping the class style in the head section of that page.