Looks like you currently have chosen 'Dark theme' or 'Default' theme and your system happens to use a dark theme. Make sure you select Light or Dark not Default system.

Sounds like you have set 'Preferences' > 'Display' > 'Formatting' > click on 'colours', changed background to black and then selected 'Override the colours specified by the content with my selections' and chose 'Always'.

But changing the background to white should still have worked.

In Config Editor:

• browser.display.document_color_use
• set Value as 0 to turn off Always and set as 'Only with High Contrast themes'.
• Or set Value as 1 to set as 'Never'
• click on OK

• browser.display.background_color
• Set String Value as #ffffff to change to white.
• click on OK

Sorry browser went squiffy and forced a resend. So if you got it twice, it's not my fault :)

If you cannot see anything to change those settings, then you will need to edit the 'prefs.js' file.

• Exit Thunderbird this is important.
• Access 'profile name' folder.
• Make a copy of 'prefs.js' as a temp - save on desktop - just in case :)
• open 'prefs.js' using a text editor.

Change the #000000 to #ffffff

• user_pref("browser.display.background_color", "#ffffff");

Save file.

start Thunderbird.

Then go and change the 'Override the colours specified by the content with my selections' and choose 'Only with High Contrast themes' or 'Never'.