You can enable the "System Theme - auto 1.3" in the Add-ons Manager > Themes....

This has been a problem on Mac for years through many, many versions (still exists 128.9.1esr (64-bit)).

A workaround that might do it for you is "Format", "Page Colors and Background", then select "Use custom colors" radio button.

Sadly this must be done each time you reply.

It is possible is has to do with the senders HTML from their mail client, some references to "Outlook being non standard" and "we aren't going to fix their problems" have been made in the past by TB development. IMHO that's bunk, this IS a TB bug period. Also in this circumstance, the text color selection process does not (appear to) work either. Sometimes your text color selection doesn't look like it does anything, but the recipient gets the changes, however they are not rendered for you ! Some less skilled developer has worked on this code a long while back and made it dis-functional.

I have used TB for 2 decades, and the addition of "enhancements" has reduced code quality. Search works sometimes, but not reliably. I understand the issues with developer resource/experience, cross platform, code quality etc.,and it is sad this bug has not been addressed.

I am on 128.9.1esr (64-bit), was a developer for 25 years, and cannot understand the version numbering. 2015 was at major release 36.x.x ??? That's almost 100 MAJOR version increments in a decade. How can there be significant regression and unit testing ?

Interestingly, the "background changes from white to black, so all the text disappears" symptom is "phantom" in that, it only occurs for the person doing the reply/forward. It is a rendering problem in the UI, and when the message is sent, the receiver does NOT get the colors the Replier saw. It makes the bug a little better, but not much. That SHOULD make the fix (in development easier).

I love TB, but am getting tired enough of the search and other bugs that I am nearing the point of moving off of it after 20 years :(.

I love TB, but am getting tired enough of the search and other bugs that I am nearing the point of moving off of it after 20 years :(.

I have been using Thunderbird from the beginning, and before I used Netscape Communicator and Mozilla Suite, always on Macintosh computers. I never activate the dark theme, but to protect my eyes I colour white backgrounds in the main window using CSS codes in my userChrome.css file Here are my settings in General > Fonts & Colours > Colours

Tangential point: The version numbering scheme is based on Firefox. Not every point release represents a major upgrade.