User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0
Steps to reproduce:
I have Manjaro Linux on to which Thunderbird 91.1.0 just arrived. When changing themes from the OS (between dark and light), the switch is not respected within Thunderbird. In 78.x (the latest before 91), the system theme that's enabled, respected the theme change from the OS properly.
When I changed from dark to light and vice versa, everything within thunderbird stayed the same.
When switching theme on behalf of the OS, Thunderbird should follow suit with using its enabled "System theme" option.