New Lisp manual nodes, Applying Customizations and Custom Themes.

New Lisp manual nodes, Applying Customizations and Custom Themes.

* doc/lispref/customize.texi (Applying Customizations):
(Custom Themes): New nodes.

* doc/lispref/display.texi (Defining Faces): Reference custom-set-faces.

* doc/lispref/modes.texi (Defining Minor Modes, Defining Minor Modes):
* doc/lispref/os.texi (Startup Summary): Copyedits.

* doc/emacs/custom.texi (Creating Custom Themes): Add reference to Custom
Themes node in Lisp manual.

* lisp/custom.el (custom-theme-set-variables): Doc fix.