How to use styleSheet.css for admin interface

Hi all,
readin this answer: is not clear if I can use the uploaded CSS
, styleSheet.css in …/public/journals/.

I upload this css:
.pkp_page_title {
background-color: #378fb7;
But nothing changes.
Do I need to use a Child Theme to change admin interface ?

I use OJS 3.1.1

Probably the uploaded css file is only used on the public site so I would try using a child theme like it is instructed there by Nate.