If you need it for your development environment you can enable `settings.local.php` in your `settings.php` 

    if (file_exists($app_root . '/' . $site_path . '/settings.local.php')) {
      include $app_root . '/' . $site_path . '/settings.local.php';
    }

and than make sure ONLY in your local `settings.local.php` (not on your live enviroment) the following settings are used

    $config['system.performance']['css']['preprocess'] = FALSE;
    $config['system.performance']['js']['preprocess'] = FALSE;