s/gtk-application-prefer-gimp-dark-theme/gtk-application-prefer-dark-theme/
I'm actually not sure if this property is supposed to be in the gtkrc
file. It looks like it is available only from GTK+3, and isn't it more
of an application settings in order to select the right theme variant,
rather than a specific theme settings?
Anyway let's at least set the right property name.