Whilst its nice to have a selection of themes out the box. The main point is to make your site different from everybody elses.
So what you need to do is to open some sample HTML files in Dreamweaver/Expression and understand what CSS changes are needed.
I would use something like TopStyle4 what assumes you can open a index.htm file that uses your CSS and you can edit the CSS and see the effect of your changes as you go.
However, you also use XML files that move parts around the page in different themes, esp for web parts. So these need to be documented.
The risk I am seeing is that I can make changes and then along comes Ver8 and I cannot upgrade or I cannot change from one existing theme from my custom theme.
It is reasonable to be able to give to a Creative a zip file of the HTML/CSS/XML and images so that they can see how to integrate their designs. Where as others might want to visit TemplateMonster.com and use one of those in this framework.
What do you suggest.