With GTK+3, high or low density is taken care by the screen scale
factor. Having a separate preferences for this is redundant, and likely
even wrong.
It may be interesting though to have a new preference later, which would
allow smaller scale icon sizing since some people like their icon
slightly smaller or bigger, and also when you are using screens whose
density is at a limit scale factor. Right now, this can be done through
themes, but a GUI settings may be interesting. If I add such feature,
let's just do it from scratch later.