This option wasn't available in 2018, when NightOwl was released. I had to test our application's Dark Mode implementation and NightOwl was super-useful then. I'm glad Apple made it easier to toggle dark mode in the interim.
Another very simple way is to make your own thing with the default Automator app.
* open up Automator and create new application
* select “change system appearance” and select toggle light/dark mode.
* save the ‘app’
Now, whenever you want to toggle light/dark mode, just open up spotlight and open up whatever you named the app.
There’s probably a way to do it with Shortcuts too.
If toggling between modes is all that is needed, it can be done right through BetterTouchTool. I just assigned a right-click+option+cmd globally for it and it works like a charm
Settings > Control Center > Display > Always Show in Menu Bar
I'll concede it takes a second click to toggle dark mode, but you also have night shift as an option, and it's 100% native.