and yet I hasve exactly that, a highlight of the selected message. But the colours usaed in those are directly derived from the operating system and most folk that report these types of error do not use a default operating system theme.

Check in Thunderbird themes and try the light and dark themes as opposed to the default theme which is the one that listens to the operating system for the highlight colour.

Thank you for your reply, Matt,

I'm using Thunderbird's dark theme and tried the default theme and I tried various ways to set the highlight or tag color for the title of the message in the list, but it doesn't show up, it's all black background with white font. This worked properly as recently as a week ago, I think, but it's getting pretty annoying.

Searching for "highlight" in this context brought up a lot of hits for using the highlighter tool and other unrelated posts about background colors, but essentially nothing about this particular problem.

Does the same issue happen if you restart Thunderbird via Help > Troubleshoot Mode...?

OK, that worked, now how do I find out what caused the problem with the selected message highlight?

Thank you!