1.- Create a css file in your custom theme where you want to add the styling.

2.- Implement the admin_enqueue_scripts action in a custom plugin or the functions.php file of your theme, like this:

/**
 * @return void
 */
function admin_style() {
	wp_enqueue_style('admin-styles', get_stylesheet_directory_uri().'/assets/css/admin-style.css');
}
add_action('admin_enqueue_scripts', 'admin_style');