If Emacs is compiled with GTK+ toolkit support, the simplest way to customize its GTK+ widgets (e.g., menus, dialogs, tool bars and scroll bars) is to choose an appropriate GTK+ theme, for example with the GNOME theme selector.
In GTK+ version 2, you can also use GTK+ resources to customize the appearance of GTK+ widgets used by Emacs. These resources are specified in either the file ~/.emacs.d/gtkrc (for Emacs-specific GTK+ resources), or ~/.gtkrc-2.0 (for general GTK+ resources). We recommend using ~/.emacs.d/gtkrc, since GTK+ seems to ignore ~/.gtkrc-2.0 when running GConf with GNOME. Note, however, that some GTK+ themes may override customizations in ~/.emacs.d/gtkrc; there is nothing we can do about this. GTK+ resources do not affect aspects of Emacs unrelated to GTK+ widgets, such as fonts and colors in the main Emacs window; those are governed by normal X resources (see X Resources).
The following sections describe how to customize GTK+ resources for Emacs. For details about GTK+ resources, see the GTK+ API document at https://developer.gnome.org/gtk2/stable/gtk2-Resource-Files.html.
In GTK+ version 3, GTK+ resources have been replaced by a completely different system. The appearance of GTK+ widgets is now determined by CSS-like style files: gtk-3.0/gtk.css in the GTK+ installation directory, and ~/.themes/theme/gtk-3.0/gtk.css for local style settings (where theme is the name of the current GTK+ theme). Therefore, the description of GTK+ resources in this section does not apply to GTK+ 3. For details about the GTK+ 3 styling system, see https://developer.gnome.org/gtk3/3.0/GtkCssProvider.html.