As @JSWolf says, just let the unit power off (by closing a sleep cover or timing out) and the state will be preserved.
Meanwhile, I suggest you switch to Nickel Menu for easily toggling of night mode; I've tried every method and it's the easiest one to use. See the #1 post in the thread linked below, plus my specific post in that thread:
https://www.mobileread.com/forums/sh...rt#post4030876