Does there exist a theme plug-in or add-on that can make the Thunderbird on macOS looks mostly dark or black?
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
Search their themes on their website. https://addons.mozilla.org/en-US/thunderbird/themes/