You're welcome gkj,
That's a wide open question, without knowing how far you want to go! Broadly speaking, as WebAssist used most (if not all) of their own extensions and solutions to create this ready-to-go solution, then having access to these tools could prove useful.
It's worth mentioning though, that although all the products carry guides and extra documentation, the documentation for the 'solutions' themselves (i.e. not the extensions) may not cover everything you might want it to if you were planning an extreme function/design mashup, because they are largely designed (and priced) to upload and go with minimal modification. I've seen on the forum somewhere someone saying that the new 'Theme-It' extension is a handy tool at this stage, although I haven't played around with it yet. I've also seen mentioned in the forum that people using CSSSculptor and SiteAssist Pro to restyle the pages didn't have much luck with those extensions in this scenario.
My advice for customisation of the design would be to do just a basic colour/logo/graphics modification, upload to the live site and get used to the functionality and make mental notes of which bits of design need adapting and how. In the mean time you could run a copy on your local machine using a testing server and do your styling and tweaking to see what works best. The best tool for this is a knowledge of CSS, for which there are loads of tutorials (Google) and a lot of knowledgeable people in this forum who can help.
The extensions themselves, are well documented with Getting Started Guides, Support Resources, Solution Recpes, How-tos and FAQs as required.
Not sure if any of this helps, but post back with specific queries as and when you need to, someone will be along to help.
Just to be clear at this stage, I'm a big fan of most (but not all) of the WebAssist products, particularly those that have nudged me towards the development side of things. A whole new layer of opportunity has opened up for me because of them, but I am still (especially compared to the WA Guys and experts here) a comparative beginner. This is a great way to learn stuff.