code: public function process(ContainerBuilder $container)
      {
          $twigFilesystemLoaderDefinition = $container->findDefinition('twig.loader.filesystem');
          $twigFilesystemLoaderDefinition->setClass(FilesystemLoader::class);
  
          if (false === $container->has('templating')) {
              $twigFilesystemLoaderDefinition->replaceArgument(0,
                  $container->getDefinition('liip_theme.templating_locator'));
              $twigFilesystemLoaderDefinition->replaceArgument(1,
                  $container->getDefinition('templating.filename_parser'));
              $twigFilesystemLoaderDefinition->setArgument(2, new Reference('kernel'));
              $twigFilesystemLoaderDefinition->setArgument(3, new Reference(SettingsRouter::class));
          }
  
          $twigFilesystemLoaderDefinition->addMethodCall('setActiveTheme', [new Reference('liip_theme.active_theme')]);
      }
docs: You can modify the container here before it is dumped to PHP code.
  
  @param ContainerBuilder $container