Since Emacs 24.3 the default X toolkit is Gtk+ version 3 (the GIMP ToolKit). This uses a configuration system based on a CSS-like syntax.

Previously, Emacs 23.1 made the default X toolkit "The GIMP ToolKit", Gtk+ version 2, and there is some documentation at gnu.org on how to configure this for Emacs.

However, from Emacs 24.3 this changed to Gtk+ version 3 which has a completely different configuration system based on a CSS-like syntax. There is generic documentation for gtk3, but no simple examples for configuring Emacs.