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>