Are you talking about Night Shift, to change the white balance of your screen to be more yellow, or Dark Mode, which can be turned on permanently without scheduling?
It sounds like you’re trying to schedule Dark Mode to stay on all the time, which can be accomplished by choosing “Dark” in System Settings-> Appearance in Ventura or later, or in System Preferences-> General in Monterey or earlier.
Thanks for clarifying! You can modify your display so it looks that way all the time by changing the white point. Most designers use 6500K but you probably are looking for something near 3500-4000K. You can do this by calibrating your monitor to create a new custom display profile. Hope that helps!
