Installation at 51% Module 'Magento_Theme' Error in Magento 2

You need to perfact solution please edit below function on line no 96 follow this- vendor\magento\framework\Image\Adapter\Gd2.php--

private function validateURLScheme(string $filename) : bool
  {
      $allowed_schemes = ['ftp', 'ftps', 'http', 'https'];
      $url = parse_url($filename);
      if ($url && isset($url['scheme']) && !in_array($url['scheme'], $allowed_schemes) && !file_exists($filename)) {
          return false;
      }

      return true;
  }

more info please refer below github url https://github.com/magento/magento2/issues/28055