I've been looking at the remove_action() call, which works for some elements, but I'm not sure it works for the CSS in the header. I could manually edit the place where the CSS is loaded, but I'd rather do it through a plugin.
wp_admin_css_color
wp_admin_css
these are hooks that supposedly work, but I can't seem to figure out how use remove_action() on the hooks.
This covers removing actions for themes: Function Reference/remove action « WordPress Codex. Don't know how relevant it is for WP's admin area.
I actually came up with a solution to this, after much searching and trial and error. To my knowledge, it's not in the WordPress codex anywhere, and when I brought it up in the WordPress IRC channels, some scoffed, as if the idea was just ridiculous. I'll try and document it later when I have a chance, but I'll start by saying it involves creating your own functions.php file in your theme, and adding several instances of wp_deregister_style(), as well as a wp_admin_css hook to override the default theme.
Interestingly, I could not get this fix to work in a plugin, which was alright for my purposes since I have a highly modified theme, but I could imagine people wanting to just do it with a simple plugin.