code: private Optional<Resource> guessMatchingBootstrap4File(
        Resource[] bootstrap4Themes, String bootstrap3ThemeFileName) {
      return Arrays.stream(bootstrap4Themes)
          .filter(bootstrap4Theme -> bootstrap3ThemeFileName.equals(bootstrap4Theme.getFilename()))
          .findFirst();
    }
docs: Guess witch bootstrap4 theme file to combine with the bootstrap 3 theme file based on the file
  name. It is assumed all resources where resolved using the "bootstrap-[theme-name].min.css"
  template.
  
  <p>Not all of the bootstrap 3 themes have a matching bootstrap 4 theme.