Rated 5 out of 5 stars

Excellent! Now that Thunderbird's updates have rendered obsolete all other coloured icon add-ons, this is the only one that works. And it works beautifully!
But note that you have to be using Thunderbird 68 (or higher, presumably) in order for it to work.
Congratulations to Dillinger for your perseverance and programming skill.

This review is for a previous version of the add-on (2.6).