You'll find most of the CSS files in the /wp-admin/css directory.
Note that, as far as I know, this is not supported behavior. So you'll need to re-install (and possibly update) all of your work with every new WordPress release. So there's not a pretty way to do this, as far as I know (like installing a new theme), but you can create new CSS files to get whatever look you want.
The biggest asset I've found in making WP easier to understand to new users is custom post types. If you organize whatever content they're going to want to post into a custom form which has only the relevant fields with more informative instructions and details about each field, most novices can figure out how to create posts pretty quickly.