This has been reported as a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1741351

On my W10 21H1, it's possible to set the Windows mode to Dark and the app mode to Light (see picture).

Are these MS Windows settings? I never changed these modes. Did the T-bird update change it?

Yes, those are Windows settings in my picture. There is a bug in 91.3.1 with some themes, although the Default theme works properly on my setup.

Many Thanks. I went to windows setup, personalization, colors, but there's no apps section as there is in your screen shot, so I change everything "light", and now I can see the text in drafts again.

So far, the only thing affected is the colors of the window tabs on the top of my screen. I don't like it, but it's better than not being able to read drafts while working on them.

I don't want W10 Light for other many reasons but only Thunderbird . It was like that till last update . So please make a new theme that allow the white body text area.

Changing Windows 10 to use a Light theme lets me read the emails I am editing but I use a signature block within a div to set a different background color for the block. That div background color (with or without the change to Windows colors) is ignored. With the unchanged Windows colors, the text was a barely read gray on the black background. With the Windows colors changed, it is black on white.

It appears that the emails I send show up correctly. I sent a test to myself at a different gmail address and it looked okay on gmail.

Today, 11/30/2021, a FIrefox Update flipped the switches in my operating system's personalized display settings removing my visual impairment settings, which caused Thunderbird to go to white backgrounds and text colors that I cannot read. After I rebooted, then the Windows 10 Explorer that shows my folders and files was ALSO switched from my visual impairment settings. This is extremely angering because no application should damage visual or hearing impairment settings!