Dark theme for Thunderbird on macOS?
It's part of the default theme package in current beta versions (running b64).

In Mojave, it switches to the dark theme automatically if you change the OS to dark. However, whereas the dark theme in Firefox is pretty much complete, the theme in Thunderbird is haphazardly applied. I'm not running dark mode Mojave specifically because Thunderbird theme is off. See bugzilla entry here: https://bugzilla.mozilla.org/show_bug.cgi?id=1486205