The Night Light feature was introduced with GNOME 3.24, and it's used to make your display switch to a warmer color in the evening, helping you sleep better and prevent eye strain.
You can enable the GNOME Night Light option from
Settings -> Displays -> Night Light -- you can choose to have it run automatically based on sunset / sunrise times or enable it on a manual schedule, and you can also control the color temperature.If you'd like to extend the Night Light functionality to automatically switch to the dark variant of the GTK theme you're using when Night Light kicks in, there's a GNOME Shell extension that does just that. When the Night Light is turned off (either by you or by the schedule you set in the GNOME Settings), the theme is switched back to the regular GTK theme variant.