It’s pretty simple. Or not. It depends on your knowledge of CSS. First I’ve splitted style.css from HTML5 Boilerplate in three parts. To be consistent with the editor of WordPress, the first part of H5bP — reset.css — is imported in markup.css witch is used for styling both the front site and TinyMCE Editor build in. Actually, it’s editor-style.css renamed.
The small second part is included within markup.css and concern global colours, headings and links. The last part is imported from style.css (root theme) and offers some tools, especially a clearfix, Media Queries and CSS Print.
I’ve put a grid.css too, with my mashup of Blueprint, 960.gs and The Simpler Grid. That why you’ll find a grid.png (940 pixels width).
Don’t be afraid to remove all stylesheets (except the first comment within style.css) to fit your need, if you want to go with a blank page!