Implemented by this commit:
commit 8de2e69f021b5b2920a07d82c9869bb394171d41
Author: Juanma Barranquero <
lekktu@gmail.com>
Date: 2019-10-06 20:53:18 +0200
Set alternate faces for some line numbers (bug#27734)
* lisp/faces.el (line-number-major-tick, line-number-minor-tick):
New faces.
* lisp/cus-start.el (display-line-numbers-major-tick)
(display-line-numbers-minor-tick): Add customization info.
* lisp/frame.el: Add `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick' to list of variables which
should trigger redisplay of the current buffer.
* src/xdisp.c (syms_of_xdisp) <display-line-numbers-major-tick>
<display-line-numbers-major-tick>: Defvar new options.
(syms_of_xdisp) <line-number-major-tick, line-number-minor-tick>:
Defsym new faces.
(maybe_produce_line_number): Use new faces for line numbers
that are multiple of `display-line-numbers-major-tick' and
`display-line-numbers-minor-tick'.
* etc/NEWS (value): Announce new feature.
* doc/emacs/display.texi (Display Custom): Describe it. Wording by
Robert Pluim <
rpluim@gmail.com>