Unfortunately, the correct way may not work for everyone. There is one additional thing that needs to be taken into consideration for anyone who uses a GTK theme changer:
Some theme changers seem to think that they have the exclusive right to modify this file, not you. When they change the theme they do not politely edit this file, they blow it away! So if you add your customizations to this file (which is exactly what it was designed for) you may awaken one morning to find that the bulldozers have come.
If you are lucky, when you go to edit the file you will see that the theme changer has planted its flag and warned you away with a "DO NOT EDIT" message. If you are luckier still, you will note that the theme changer has added an include command to load another file where your customizations are expected to live now that they have been evicted from .gtkrc-2.0.
The easiest thing to do is to just pack your things and go. Place your customizations in the other file. And you must then swear loyalty to the theme changer that evicted you. If you should ever use another theme changer or other utility that blows away .gtkrc-2.0, your customizations may be out in the cold again if the new landlord of .gtkrc-2.0 doesn't make the same provisions for loading another file, or has decided that it wants to load a file with a different name. (For instance, gtk-chtheme loads "/root/.gtkrc.mine", but lxappearance loads "/root/.gtkrc-2.0.mine".)
Alternatively you could banish the unsociable theme changers from your system, or just use them for examining themes while staying away from their "Apply" or "OK" buttons, and edit .gtkrc-2.0 manually.
