If you need to force change to admin styles for all blogs in a network, using the “/mu-plugins/” folder is likely the best route. Just create a new file in that folder named something like “admin-styles.php” with the following code — or add it to a globalfunctions.php with all modifications for the entire network. We’d been trying to do this within globally active themes for quite some time. It’s best to actually take advantage of the must use plugins directory, however!

<?php
/*
Plugin Name: Admin Styles
Description: Easy way to change styles in the admin. 
*/
add_action('admin_head', 'admin_network_styles');

function admin_network_styles() {
  echo '<style>
    #wp-admin-bar-blog-221,  /** Reference Site **/
    #wp-admin-bar-blog-156 /** Main Blog **/
    {
    	background: yellow !important;
    }
</style>';
}
?>