I was using Thunderbird 78.7.1 (64-Bit). This version is not usable for me due to critical color scheme bugs. It shows small black letters on gray background, which doesn’t work for my old eyes. I’m currently forced to use other mail programs to answer my mails.
In the Thunderbird options I’ve checked “System Colors”. My system is Ubuntu 20.04.2 LTS with Gnome 3.36.8. Installed system theme is GNOME Flashback (Metacity). Attempts to change the gray background failed.
Here is a screenshot:
Hope you can help (or fallback to previous version of Thunderbird).