customizing html/css using gp

Hey Tom, love the theme. I'm moving away from the bloat that has become X and am re-appreciating your lightweight and simple theme. Many people don't realize that it is far more complex to simplify than it is to just throw everything in. Thank you.

If I wanted to use custom HTML/CSS to build the entire site, how hard is it to do? Would I have to re-write a bunch of template files or are there only a few to tweak?

Alternatively, would you want to do it? I'm inputting the URL in the 'website url' box, even though it isn't currently running WP. It is a site up and running right now, and my client may want to fork it over to WP and GP.

Thanks!