Most people in the Inverse World setting live on the floating landmasses of the Island Ring, some of which are the size of continents. But since the sun in this world is down, below the islands, people live on the side away from the sun.
So is it dark all the time?
It seems like it would always be dark in the cities on the topsides of the islands.
If it is dark all the time in the areas where most people live, I'd expect people to be adjusted for that, like nocturnal or underground creatures in the real world: either seeing infrared light or using sonar to navigate. Either of these would be important for players to know about.
But then I read the playbook for the Rainlord, and I see that they have a special ability available (Aspect: Liquidity: Light) that prevents them from getting lost due to darkness. That makes it sound like darkness is an impediment to people in this world.
As the GM, I'm trying to figure out how to describe this world to the players. If most civilization dwells in the dark, that's a very significant piece of information. I know we could just make something up ourselves, but as a starting point, I'd really like to know how the Inverse World is normally understood.