When I think of this world, I consistently imagine the sky being yellow. But I don't want it to be so; I feel like this implies some sort of pervasive pollution. That's not what I have in mind. Even if excessive pollution was the cause or side effect of the Apocalypse, this story takes place thousands of years afterward, and the global population never reached the same level, so this hypothetical pollution would have subsided.
Well, maybe some of it remains, if the population hasn't "recovered"? Will I have to explain that aspect too? I have some ideas in mind, but I want to follow Stan Lee's advice of not getting extremely deep into the specific sciences of things. I am a little more interested in establishing the current world rather than the past, for now.