Gimp is a GTK application. As such many characteristics of the GUI are customized via a gtkrc file (fonts, colors, and some behavior when applicable), while icons/images are provided separately. In short, gtkrc+icons=theme.
Then you are on a Unix machine which is potentially used by several people (at the same time or sequentially). So there are parts of the Gimp setup that are available to all (in /usr) and parts that are specific to your userid (in your Gimp profile: ~/.gimp-2.8/). So the theme can be in either (if adding new themes, best put them in your Gimp profile, so that your backups take them in account).
Then you are on a Unix machine which is potentially used by several people (at the same time or sequentially). So there are parts of the Gimp setup that are available to all (in /usr) and parts that are specific to your userid (in your Gimp profile: ~/.gimp-2.8/). So the theme can be in either (if adding new themes, best put them in your Gimp profile, so that your backups take them in account).