If you’re running a desktop environment that is not Gnome3 but relies on GTK there is a simple way to make your applications run with the dark theme. You only need to open the following file

$ vim ~/.config/gtk-3.0/settings.ini

and change/add gtk-application-prefer-dark-theme=1 so it looks like this:

[Settings]
gtk-application-prefer-dark-theme=1

That’s all!