Setting as night?

What do you expect to happen if you had a feature to set a map to "night"? What you're looking for aesthetically might be subjective depending on who you ask. For instance, a lot of the classic 2D video games didn't darken the map so much as blue tint the entire screen for night environments. Roll20 doesn't have a switch you can flip to make a map darker or tint it crazy colors. If I wanted to change my page from day to night, I'd make a blue translucent PNG and overlay it on top of all the art assets on the Map and Background layer. For daytime, I'd just remove the PNG from the layer.This workaround unfortunately wouldn't work on your tokens, but the rest of your map would look the part.

I'd recommend creating a suggestion thread in the suggestion forum and see if other users can expand/upvote this idea. I think what might gain more appeal is not so much a night switch but allow tinting options for the entire tabletop. Like, users could choose to tint the tabletop blue for nighttime, orange/red if your players decide to run amok in a volcano or on the surface of mars, green/grey for a hazy swamp. Stuff like that.