highlighting active link on sidebar Kubrick

Hi, I posted question about this before. And got this reference which is good, except I couldn’t get it to work – maybe I was putting the code in the wrong places, I’m not sure. I tried it lots of times. Is there a simpler way or can anyone explain the info below to make a sidebar link stay highlighted while that page is active.

This codex page covers just that:
https://codex.www.ads-software.com/Dynamic_Menu_Highlighting