How to KEEP dark mode forever

The dark mode setting in System Preferences : General remains on across boots until you change it to light mode. That is my experience with Mojave 10.14.6 (18G6032), and Catalina 10.15.7 (19H15).

Did you previously install a third-party tool that allows you to control when light and dark mode change, and forgot about it?

Well, since I do not have f.lux installed, and my dark mode remains indefinite, a good test would be to either apply an update to f.lux if one is available, or thoroughly remove it, reboot, set dark mode, and wait to see if it fails to remain set.

darylngee wrote:

In Mojave OS, how does one KEEP dark mode?

I have found that when I restart, dark mode is gone and I have to reset the computer to dark mode. Is there any way to make it "permanent" or default?

What is the need to restart? Just simply let the Mac sleep. Restart only if necessary.

>System Preferences>General>Dark

Use sleep and Energy Saver on your Mac - Apple Support

How to use Dark Mode on your Mac - Apple Support