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;
    }

}