Apple - Can I turn to Dark Mode only for specific apps in macOS Mojave?

There's a command that works for this:

defaults write <Bundle-Identifier> NSRequiresAquaSystemAppearance -bool yes

It's explained here: https://www.tekrevue.com/tip/exclude-app-dark-mode-macos-mojave/

If you don't know the app's bundle id, you can query it using AppleScript, which you can run from the command line with osascript:

osascript -e 'id of app "<App-Name>"'

More free options include Gray app and NightOwl. Both can be installed using Homebrew:

brew install gray
brew install nightowl

I've been using LightsOff for a couple of days now, seems to work very well.

It does "switch" between modes when you open an app that you have excluded and you can actually see the switching which is a bit of a downside.