The standard locations for gtk2 configuration files are
* `/etc/gtk-2.0/gtkrc`: system wide
* `~/.gtkrc-2.0`: user specific
Thus, try to change one of those files instead of `~/.config/gtk-2.0/gtkrc`.
Actually there is some inconsistency, because `gtk3` is indeed using `~/.config/gtk-3.0/settings.ini`, but that is another story.