It's now on Tom's todo list for Ubooquity so until then this issue is on hold. I will however include at some point in the near future a small number of theming options as an alternative.
I had an idea about a work around until it gets implemented properly. If you were to store a copy of the folder.jpg inside of a folder.cbz file this should show up in the OPDS feed and I would be able to use it in the app!
to be honest it is not that important i wouldnt implement a workaround