You have not said what you have done to change this behavior, so I have to start from scratch with suggestions to fix this.

Thunderbird menu: Tools: Options: Display: Formatting tab. Click the "Color" tab and check the setting for the background colour.

See screenshots below for guidance.

Bruce, thanks for your reply. I was close, but not close enough! I changed the background color under Options/Composition/HTML/Background Color I followed your advice and it works as expected. Thanks again!

I have another questions about exactly how to use a scanned signature so my actual signature shows as my signature in a email. I looked up how to do it, but I wasn't able to get it to work - can you help with that as well, or do I have to post a separate question in the forum?

It would be best to post this as a separate question. The reason for this is that a separate "thread" in the forum for each issue makes it easier to find when people do searches to try to solve their own problems.