Hello and thanks again for the wonderful FEBE!
Please correct me if I'm wrong: I think that the backed up Lightweight themes (json files) do not include the actual images, but only links to addons.cdn.mozilla.net. Since Mozilla has already started removing a lot of content related to what they call "Legacy technology", perhaps there's a way to fully backup (offline) these themes, too? It'd be a pity for users to lose them overnight.
P.S. I'm not sure what occurs when backing up complete Themes (jar files). It seems like they are fully backed up.