Light Overlay
Default on = true/false

There is currently no way to do that.
I also don't really see why someone would need that since it's normally only enabled as long as you are building something and not constantly.

I will try to add that, but it will probably end up in the Mod Options / Config file.

I would like this option too. I care a lot about my skyblock safety, and having to check whether it's turned on or not to be able to distinguish it being disabled from actually no spawnable places available, is tedious and can be forgotten. I've lost items due to this. The player should be able to decide whether they want it turned on by default really.