Hi Brian, I think the white border you're seeing is a type of optical effect known as "Mach banding":
https://en.wikipedia.org/wiki/Mach_bands

Probably the best adjustment to reduce it is using a darker red color like you've already done.

I've experimented with a few other ideas but have not really come up with anything better than that.

You can set that color to come from the current theme so it can be set differently for a dark background theme versus a light background theme.

- Michael