dynamic highlight menu

Hello,
I’m trying to create a dynamic menu in which the selected button would switch color permanantly. That would ease the navigation a little as I intend to add many button in the menu. That way, anyone would know where he is in the menu.
I was really confused by this text
https://designisphilosophy.com/css/highlight-current-page-in-wordpress-menus/
which didn’t help much. I couldn’t find any of those line in my style sheet (style.css).

Could you help me please? Should I copy here the stylesheet?
Here is the website
misterwayne.com
Thanx a lot