I notice this may not affect everyone but I couldn't help but notice on a brand new fresh install of Thunderbird on MacOS if you had enabled the global dark theme Thunderbird would also inherit this theme (so far so good). Upon sending an email to someone at gmail I noticed my email appears to have sent blank! But wait! No! Thunderbird has sent my email with the text formatting preserved from my theme!
This is a horrible out-of-box experience that to the unsuspecting user they would not know that Thunderbird would send Gmail users an all white email on an all white background. This is absolutely maddening and I had to explicitly go in and tell Thunderbird to not only do not preserve formatting but also send my emails as plain text.
Unless someone is composing their emails in html I recommend that Thunderbird send emails as plain text by default and only change that behavior if they user has explicitly requested their emails be sent in html format.
