The GTK theme is the overall theme that handles stuff like the panel color, the backgrounds for windows and tabs, how an application will look when it is active vs. inactive, buttons, check-boxes, etc. In most of the desktop environments the theme can be changed or customized to a certain extent and we'll talk about one such instance where well customize gtk themes found in gnome desktop environment and apps using gimp toolkit.
See the getting started guide for instructions how to get it up and running.