Menu app icon > Options > General
   scroll to the bottom and click on 'Config Editor' button 

   It will say be careful...

   Right click in the list of preferences area and select 'New' > 'Integer'
   At prompt enter the preference name: ui.systemUsesDarkTheme
   Click on OK
   Enter the Value: 0
   click on OK 

   Close 'about:config' - top right x '

   You can also change to light mode in Tools - Add-ons.
   
   Hope this helps for now!    :)
   
   .

Hi. I understand your pain. It's a problem in the way TB interprets HTML when the system dark mode is active. From what I understand, it's a bug that's being worked on. Here's my thread - https://support.mozilla.org/en-US/questions/1295378 Try this (Thanks, Toad-Hall) : 'Whilst the bug people are working on this problem, I suggest you try the information below to help with the situation. If computer app mode = dark and using dark mode in Thunderbird, the following will force a light mode onto the Message Pane and a few other areas. But it does mean you can keep app dark mode for computer apps and use dark mode in Thunderbird and see all emails." Menu app icon > Options > General scroll to the bottom and click on 'Config Editor' button It will say be careful... Right click in the list of preferences area and select 'New' > 'Integer' At prompt enter the preference name: ui.systemUsesDarkTheme Click on OK Enter the Value: 0 click on OK Close 'about:config' - top right x '''''' '''You can also change to light mode in Tools - Add-ons.''' Hope this helps for now! :) .

thanks buddy