Admin Theme Issue

I’m trying to create an admin theme. I want it to only look at my style sheet and ignore the default WordPress one. I read the codex and it seems that this the necessary code, but when I activate the plugin, nothing happens. Am I missing something?

<?php
/*
Plugin Header up here.
*/

function my_wp_admin_css() {
  echo '<link rel="stylesheet" href="/wp-content/plugins/admin-test/wp-admin.css" type="text/css" />';
}

add_action('wp_admin_css','my_wp_admin_css');

?>