Creating WordPress Admin Themes

You’ll want to add your own stylesheet, e.g.:

add_action('admin_head', 'custom_admin_style');
function custom_admin_style() {
    wp_register_style( 'adminStylesheet', plugins_url('adminstylesheet.css', __FILE__) );
    wp_enqueue_style('adminStylesheet');
}

You can then style the admin as you like. For UI changes beyond styling its not possible to answer within the scope of this question as there are too many things possible to say in an answer like this.

For an example of what’s possible and some of the pitfalls and advantages of theming the admin area, see this article by Noel Tock:

http://www.noeltock.com/web-design/wordpress/wp-admin-is-an-experience-too/