Since my attentions on roundcube currently, I decided to look into some plugs. One thing that I would like be able to do is highlight messages in the list so they get a color background, and I found a plugin to do this, cor/message-highlight but after installing it, I was unable to access the roundcube settings in the GUI (it would time out after a long period of loading any settings), so I removed it (everything is back to normal operation now).
I don’t see anything else that might do this, however.
There is a label plugin called weird-birds/thunderbird_labels but I don’t want labels, I want color highlighting.