If you want the stylesheets registered in the backend, use the admin_enqueue_scripts
action.
function wp94156_register_styles() {
wp_register_style(
'your-stylesheet',
get_stylesheet_directory_uri() . '/your-stylesheet.css',
$deps, // optional
$version // optional
);
// since you only want it registered, we won't enqueue it
}
add_action( 'admin_enqueue_scripts', 'wp94156_register_styles' );