A true black (instead of dark gray) darkmode that saves power by turning off the pixels on OLED displays would be great. The old versions of K-9 Mail had that.
Yeah, I was just looking for this setting and found this thread by searching for a way to switch Thunderbird to and OLED theme. Sucks that it's not a thing.
This is a pretty standard feature on a lot of apps nowadays. I too went looking for the setting but alas, it doesn't exist. This could simply be implemented in the same way the Droid-Ify app did. In Display > Theme, have an extra option called "System Black" so that it will still follow the user's dark mode setting but will be true black rather than grey.