If you want to toggle between light and dark mode, it can be done with a single shell command:
osascript -e 'tell app "System Events" to tell appearance preferences to set dark mode to not dark mode'
Try it and it will switch the mode immediately. No need to restart or install anything.
How does it work?
It’s AppleScript. dark mode is a boolean value in the user defaults system. not dark mode is the opposite of that value. So, for example, if the value is true, it’s like saying not true (i.e. false), effectively acting like a light switch.
Enjoy the dark!