Appendix D X Options and Resources
You can customize some X-related aspects of Emacs behavior using X
resources, as is usual for programs that use X. On MS-Windows, you
can customize some of the same aspects using the system registry.
See MS-Windows Registry.
When Emacs is built using an `X toolkit', such as Lucid or LessTif,
you need to use X resources to customize the appearance of the
widgets, including the menu-bar, scroll-bar, and dialog boxes. This
is because the libraries that implement these don't provide for
customization through Emacs. GTK+ widgets use a separate system of
`GTK resources', which we will also describe.