class ThemeInstallController

class ThemeInstallController extends ControllerBase {
    
    /**
     * The theme installer service.
     *
     * @var \Drupal\Core\Extension\ThemeInstallerInterface
     */
    protected $themeInstaller;
    
    /**
     * Constructs a new ThemeInstallController.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer
     *   The theme installer.
     */
    public function __construct(ConfigFactoryInterface $config_factory, ThemeInstallerInterface $theme_installer) {
        $this->configFactory = $config_factory;
        $this->themeInstaller = $theme_installer;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('theme_installer'));
    }
    
    /**
     * Install a theme as default.
     *
     * @param string $theme
     *   The theme to install as the default theme.
     *
     * @return array
     *   A render array confirming installation.
     */
    public function installDefault(string $theme) {
        return $this->installTheme($theme, 'default');
    }
    
    /**
     * Install a theme as the admin theme.
     * @param string $theme
     *   The theme to install as the admin theme.
     *
     * @return array
     *   A render array confirming installation.
     */
    public function installAdmin($theme) {
        return $this->installTheme($theme, 'admin');
    }
    
    /**
     * Installs a theme.
     *
     * @param string $theme
     *   The theme to install.
     * @param string $default_or_admin
     *   Which type of theme to install, can be `default` or `admin`.
     *
     * @return array
     *   A render array confirming installation.
     */
    private function installTheme($theme, $default_or_admin) : array {
        assert(in_array($default_or_admin, [
            'default',
            'admin',
        ]), 'The $default_or_admin parameter must be `default` or `admin`');
        $config = $this->configFactory
            ->getEditable('system.theme');
        $this->themeInstaller
            ->install([
            $theme,
        ]);
        $config->set($default_or_admin, $theme)
            ->save();
        return [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'theme-installed',
            ],
            '#markup' => "Installed {$theme} as {$default_or_admin} theme",
        ];
    }

}