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If you use a Desktop Environment, it's likely that it provides a tool for changing the GTK theme, without needing a configuration file, and providing the ability to apply themes immediately without restarting applications.
Alternatively, you can use third-party GUI tools such as
gtk-chtheme which just write to the configuration files.
Otherwise, you can manually edit the configuration files yourself. For GTK+ 2.x, it is
~ being your home directory. For system-wide configuration,
gtk-font-name="Arimo 9" gtk-theme-name="Adwaita" gtk-icon-theme-name="Faenza"
For GTK+ 3.x, the file is located at
/etc/gtk-3.0/settings.ini. The syntax differs a little:
[Settings] gtk-font-name=Arimo 9 gtk-theme-name=Adwaita gtk-icon-theme-name=Faenza