Mint 21.1 Cinnamon - 'selected' button outline hard to see

[OneSkate]

This is, I believe, my first post to this forum... so I hope I'm not in violation of community standards. I have searched around to see if the question has already been asked / answered. If it has been, I was unable to find it.

I previously used Mint 20 Cinnamon, which had the same issue:

I find the 'selected' button outline - that is, the outline that appears around a button when said button is selected - to be almost invisible. It's maybe one pixel wide, and hard to see even in HighContrast theme. I use a TV display on the other side of the room for videos, but even with my laptop actually atop my lap it's still hard to make out.

I'd be grateful for any hints as to how I might beef up that button outline for greater visibility.

Cheers!