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 other option would be to create a plugin that makes some changes to the WordPress admin panel after-the-fact using Javascript. This may not be a very elegant solution, but it might work if you’re just trying to re-style a couple of buttons.
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.