Controller/ThemeController.php
public function changeAction()
    {
        $theme = $this->container->get('request')->request->get('admin_theme');
        $this->container->get('vince_t.admin.theme.handler')->setCurrentTheme($theme);
        $headers = $this->container->get('request')->server->getHeaders();
        $referer = $headers['REFERER'];
        return new RedirectResponse($referer);
    }
public function changeAction()
    {
        $theme = $this->container->get('request')->request->get('admin_theme');
        $this->container->get('vince_t.admin.theme.handler')->setCurrentTheme($theme);
        $headers = $this->container->get('request')->server->getHeaders();
        $referer = $headers['REFERER'];
        return new RedirectResponse($referer);
    }
Change the current theme used

@return RedirectResponse