Globally register styles but enqueue them selectively

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' );