Hey again,
So I just uploaded a new version that should fix this. The issue is, apparently, the darkness setting on the new 0.18 API can't be applied while the surface is 'frozen' (which happens for both the grace period, and then again to keep the night going); I've reorganized the code to ensure it gets applied at the right time.
For an existing save, please go into the setting and turn off "pitch black", then exit the menus to the game unpauses for a second, and go in and turn it back on. This should get it working and it should work out of the box for all future saves.
Thanks again and have fun.