Wordpress - CSS rules for admin area

functions.php

//this goes in your functions.php file
<?php //this line is only for the Gist syntax highlight, don't need to put it in your file  
function my_admin_styles() {
  wp_enqueue_style('admin_styles' , get_stylesheet_directory_uri().'/admin-style.css');
}

add_action('admin_head', 'my_admin_styles');