I finally decided to try Thunderbird 68.1. (I get Thunderbird from Mozilla directly and untar the package after verifying the signature. This process makes it easier to back out.)
Upon launch, I only get dark windows, regardless of the application appearance settings in Gnome Tweaks. I will spare everyone my debugging steps, but I am guessing the prereq for GTK3 not yet being met in PureOS is causing my problem. Thunderbird 68 requires GTK 3.4; I have PureOS with amber updates at 3.24.5. (This is the same level as Buster.) I have not traced it to a particular item, but I am not going to spend any more time due to being down-level from the requirement.
I would be interested if anyone else has been successful; otherwise, I’ll bide my time.