my_module.my_service:
  class: Drupal\my_module\MyService
  arguments: [ '@config.factory' ]

class MyService {

  /**
   * The Config Factory Service.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  private $configFactory;

  /**
   * Constructs the Example service.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $configFactory
   */
  public function __construct(ConfigFactoryInterface $configFactory) {
    $this->configFactory = $configFactory;
  }

}

function my_module_example() {
    $myService = \Drupal::service('my_module.my_service');
}

namespace Drupal\my_module;

class MyModuleServiceProvider extends ServiceProviderBase {

  /**
   * {@inheritdoc}
   */
  public function alter(ContainerBuilder $container) {
    $definition = $container->getDefinition('some_core_service');
    $definition->setClass('Drupal\my_module\MyCoreOverrideService');
  }

}

namespace Drupal\my_module;

class MyCoreOverrideService implements SomeCoreServiceInterface {

  public function reimplement() {
    // ...
  }

  public function every() {
    // ...
  }

  public function interface() {
    // ...
  }

  public function method() {
    // ...
  }

}

namespace Drupal\my_module;

class MyCoreOverrideService extends SomeCoreService {

  public function onlyTheOverriddenMethod() {
    // ...

    $someDataFromTheOriginalService =
      parent::methodFromTheOrginalService();
  }

}

services:
  my_module.core_service_decorator:
    class: Drupal\my_module\CoreServiceDecorator
    arguments: [ '@my_module.core_service_decorator.inner', '@config.factory']
    decorates: core_service

namespace Drupal\my_module;

class CoreServiceDecorator implements SomeCoreServiceInterface {

  public function __construct(
    CoreServiceInterface $coreService,
    ConfigFactoryInterface $configFactory
  ) {
    $this->coreService = $coreService;
    $this->configFactory = $configFactory;
  }

  public function method() {
    $coreServiceData = $this->coreService->method();

    return $data;
  }

}

namespace Drupal\my_module;

class CoreServiceDecorator implements SomeCoreServiceInterface {

  public function proxy() {
    return $this->coreService->proxy();
  }

  public function every() {
    return $this->coreService->every();
  }

  public function interface() {
    return $this->coreService->interface();
  }

  public function method() {
    return $this->coreService->method();
  }

}

services:
  my_module.core_service_decorator:
    class: Drupal\my_module\CoreServiceDecorator
    arguments: [ '@my_module.core_service_decorator.inner', '@config.factory']
    decorates: core_service
    decoration_priority: 5

services:
  ie9.css.collection_renderer:
    class: Drupal\ie9\Asset\CssCollectionRenderer
    arguments: [ '@state' , '@ie9.css.collection_renderer.inner' ]
    decorates: asset.css.collection_renderer

namespace Drupal\ie9\Asset;

class CssCollectionRenderer implements AssetCollectionRendererInterface {

  public function __construct(StateInterface $state, AssetCollectionRendererInterface $cssCollectionRenderer) {
    $this->state = $state;
    $this->originalCssCollectionRenderer = $cssCollectionRenderer;
  }

  public function render(array $css_assets) {
    if (count($css_assets) <= 31) {
      return $this->originalCssCollectionRenderer->render($css_assets);
    }

    // Render for IE9...

    return $elements;
  }
}