class BlockTheme

class BlockTheme extends ProcessPluginBase implements ContainerFactoryPluginInterface {
  
  /**
   * Contains the configuration object factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;
  
  /**
   * Contains the system.theme configuration object.
   *
   * @var \Drupal\Core\Config\Config
   */
  protected $themeConfig;
  
  /**
   * List of themes available on the destination.
   */
  protected $themes;
  
  /**
   * Constructs a BlockTheme object.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin ID for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
   *   The migration entity.
   * @param \Drupal\Core\Config\Config $theme_config
   *   The system.theme configuration factory object.
   * @param array $themes
   *   The list of themes available on the destination.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, Config $theme_config, array $themes) {
    parent::__construct($configuration, $plugin_id, $plugin_definition, $migration);
    $this->themeConfig = $theme_config;
    $this->themes = $themes;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
    return new static($configuration, $plugin_id, $plugin_definition, $migration, $container->get('config.factory')
      ->get('system.theme'), $container->get('theme_handler')
      ->listInfo());
  }
  
  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    [
      $theme,
      $default_theme,
      $admin_theme,
    ] = $value;
    // If the source theme exists on the destination, we're good.
    if (isset($this->themes[$theme])) {
      return $theme;
    }
    // If the source block is assigned to a region in the source default theme,
    // then assign it to the destination default theme.
    if (strtolower($theme) == strtolower($default_theme)) {
      return $this->themeConfig
        ->get('default');
    }
    // If the source block is assigned to a region in the source admin theme,
    // then assign it to the destination admin theme.
    if ($admin_theme && strtolower($theme) == strtolower($admin_theme)) {
      return $this->themeConfig
        ->get('admin');
    }
    // We couldn't map it to a D8 theme so just return the incoming theme.
    return $theme;
  }

}