Found this thread How to enable dark mode in Casper 3.0? where @DavidDarnes mentions that it should work automatically.
Just to make sure, I am talking about the front end, not the manual switch of the admin backend.
Edit:
Now I am super confused.
I tried it with https://demo.ghost.io/ just as @DavidDarnes showed in his post. But it doesnt work as well for me.
