How to change the color of a clicked link in a side navigation menu?

Site URL: https://bamboo-stingray-l426.squarespace.com/config/

I've created a side navigation menu in a 'Meet the Team' page. I'm a total beginner at this, so how do I change the color (from gray to green) once a specific link has been clicked? In my code right now, the links are clickable and I know how to link it, I just don't know how to make it change it's color from gray to green? BTW, the link is redirected to the same page but different content.
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Hi. Your site is private. Can you setup password & share url again? We can check easier