load CSS only for administrators on backend

This is what I use, tested and works…

function wpse_admin_styles(){
    $user = wp_get_current_user();

    if( !empty($user) && count(array_intersect(["administrator"], (array) $user->roles ))) {
        wp_enqueue_style(
            'admin_css', 
            get_stylesheet_directory_uri() . '/css/admin-styles.css', array(), filemtime( get_stylesheet_directory() . '/css/admin-styles.css') 
        );
    }
}

add_action('admin_enqueue_scripts', 'wpse_admin_styles');