Thank you Professor Kitchin. Would something like follows would work?
#+BEGIN_SRC emacs-lisp :results silent
(add-hook 'org-mode-hook (lambda () (font-lock-add-keywords
                                     nil
                                     '(("^-\\{5,\\}"  0 '(:foreground "red"
:weight bold))))))
#+END_SRC

-Alper



On Tue, Sep 28, 2021 at 8:18 PM John Kitchin <jkitchin@andrew.cmu.edu>
wrote:

> you can add a rule like this in an org-mode hook:
>
> #+BEGIN_SRC emacs-lisp :results silent
> (font-lock-add-keywords
>  nil
>  '(("^-\\{5,\\}"  0 '(:foreground "red" :weight bold))))
> #+END_SRC
>
> that will make a line starting with at least 5 - be red and bold in color.
>
> John
>
> -----------------------------------
> Professor John Kitchin (he/him/his)
> Doherty Hall A207F
> Department of Chemical Engineering
> Carnegie Mellon University
> Pittsburgh, PA 15213
> 412-268-7803
> @johnkitchin
> http://kitchingroup.cheme.cmu.edu
>
>
>
> On Tue, Sep 28, 2021 at 4:00 AM Eric S Fraga <e.fraga@ucl.ac.uk> wrote:
>
>> You could use hi-lock-mode to define your own colouring for such
>> lines.  But I'm sure those that under font-locking better might be able
>> to add appropriate rules for this horizontal line construct.
>>
>> --
>> : Eric S Fraga via Emacs 28.0.50, Org 9.5-g9a4a24
>> : Latest paper written in org: https://arxiv.org/abs/2106.05096
>>
>>