User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:90.0) Gecko/20100101 Firefox/90.0

Steps to reproduce:

A number of bugs indicate that Thunderbird is poorly prepared to navigate the new universe where Mac, Linux, and Windows UIs can be switched into "dark" mode. The following bugs appear to be related:
bug 1544212
bug 1638233
bug 1640390
bug 1651617
bug 1663275
bug 1675862
bug 1685781
bug 1718772
bug 1720640
bug 1705497
bug 1708974
bug 1709489
bug 1712632
bug 1657833
bug 1614731
bug 1667561

In the case of that last bug, this becomes a user security issue as it is hard for the user of OpenPGP to see whether they are sending a message encrypted or not.

you might find a few others in this query https://mzl.la/3ko7NZd

Batched by OS:

Linux:
bug 469300, bug 1455175, Bug 1544212 , Bug 1638233, bug 1659953, Bug 1663275 , bug 1671889, bug 1690531, bug 1694776, bug 1708974, bug 1720640, Bug 1720894, bug 1721264

Mac:
bug 1624714, bug 1651617, bug 1657833, bug 1667561, bug 1675862

Windows:
bug 1605727, Bug 1640390, Bug 1614731, bug 1669373, bug 1683575, bug 1685781, bug 1691316, bug 1689165, Bug 1691316, Bug 1705497, bug 1709489, Bug 1712632, bug 1718772