Just use a draw rectangle over the whole screen with transparency. Search in the forums for and play with zim.qst for an example if you like. I wrote my own clock, time based events, nighttime, clouds, and rain scripts. There is even a lantern that works like lttp in there but it is slightly buggy only because I didn't bother to make special overhead tiles that stay dark because Link is under them at night.
Making extra dmaps is very impractical because then you'd have to write extra scripts that trigger secrets on every map when triggered on one, which is suitable, but so is making a black transparency rectangle over the whole screen.
Search YouTube for Zim Zelda Classic Nighttime for a short video.