Hello, I'm trying version 3.0.4 and I think it's convenient to solve line 24 to see the screenshot
vendor\croogo\croogo\Extensions\src\Template\Admin\Themes\index.ctp
$file = $this->Url->assetUrl($currentTheme['name'] . '.' . $currentTheme['screenshot']);
$file = $this->Url->assetUrl($currentTheme['name'] . '.' . $currentTheme['screenshot'],array('fullBase' => true));