Dark Horse downloader 
This Chrome extension gives you the option to download Dark Horse comic books from your digital library. You can download it here.
Usage
Go to your digital library page (gallery view must be selected), and click on the download icon next to the comic book you want to save.
Converting comics into CBZ format
Comics are downloaded as a tar archive that is not compatible with other comic book readers, so you have to convert them using Dark Horse converter before you can open them in the reader of your choice.