I altered the background buildings slightly, adding some domes so it seems like a continuous theme and tried to make some of the buildings look more intact. I also added bounce lights and rim lights to the foreground and mid-ground as you suggested but I'm not too sure how effective it is.

I think I still need to revise the building designs to make them look more stable and functional.

Originally, I thought the background was a bit dull and empty so I added a second, smaller sun and thought it would be okay seeing as Calvino doesn't outright say that these cities are on our planet from what I remember. If it needs to go, please let me know.