Never having seen or heard of gtk2_prefs before, on this occasion trying it out worked as expected.
It does
not require to be ran as root.
Upon running gtk2_prefs creates ~/.gtkrc-2.0, and a default theme seems to get applied already without any further manipulation.
All that is required is to restart the gtk apps in question.
BTW, very happy with this, getting rid of those huge screen estate wasting icons, e.g. in rox filer.