After upgrading my MacBook to OS X Big Sur, the font that I use for viewing plan text messages has reverted to a proportional-width font. Before the upgrade, I had used a fixed-width font for plain text messages, which is important for viewing tables, etc. However, after the upgrade, I can't find out or remember how to put it back. The font for composing plain text messages is a fixed-width font (under Latin text font in the Font preferences window), but how can I read messages in a fixed-width font?
I'm sure I did this change somehow in OS X Catalina, but I cannot remember how or where.