Hook to run afer config import I need to run some code after the config for my module is imported(it will create/update a css file that depends on configuration). So what is the correct hook to use? The module install will run only on first time when the module is instaled.
The config system has switched already to events and the documentation for events describes in "My First Drupal 8 Event Subscriber" how to subscribe to a config event:

https://www.drupal.org/docs/8/creating-custom-modules/subscribe-to-and-dispatch-events

See also this example from the Color module:

/core/modules/color/src/EventSubscriber/ColorConfigCacheInvalidator.php

&lt;?php

namespace Drupal\color\EventSubscriber;

use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
use Drupal\Core\Config\ConfigCrudEvent;
use Drupal\Core\Config\ConfigEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * A subscriber invalidating cache tags when color config objects are saved.
 */
class ColorConfigCacheInvalidator implements EventSubscriberInterface {

  /**
   * The cache tags invalidator.
   *
   * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
   */
  protected $cacheTagsInvalidator;

  /**
   * Constructs a ColorConfigCacheInvalidator object.
   *
   * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
   *   The cache tags invalidator.
   */
  public function __construct(CacheTagsInvalidatorInterface $cache_tags_invalidator) {
    $this-&gt;cacheTagsInvalidator = $cache_tags_invalidator;
  }

  /**
   * Invalidate cache tags when a color theme config object changes.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The Event to process.
   */
  public function onChange(ConfigCrudEvent $event) {
    // Changing a theme's color settings causes the theme's asset library
    // containing the color CSS file to be altered to use a different file.
    if (strpos($event-&gt;getConfig()-&gt;getName(), 'color.theme.') === 0) {
      $this-&gt;cacheTagsInvalidator-&gt;invalidateTags(['library_info']);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[ConfigEvents::SAVE][] = ['onChange'];
    $events[ConfigEvents::DELETE][] = ['onChange'];

    return $events;
  }

}