I didn't know about this and now I am really interested in the update. I've been using one of the overlay ones for a while and it is kind of annoying.
Just swipe down from the top of the screen to reveal the quick settings, and then press and hold the Settings icon (gear) for a few seconds and release. This will enable the UI tuner.
Swipe down from the top again, then tap "EDIT" in the bottom right corner. There should be a tile for "Night mode", which you can drag and drop into the quick settings panel for easy toggling.