Admin Theme Problem

I’m trying to write a WordPress admin theme plugin. I want it to only look at my stylesheet and not the WordPress default. This seems like the correct code to me, but when I activate the plugin, it doesn’t work.

Plugin Header

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



Any ideas on why this isn’t working?