From your post, I'm not sure of the issue. Is everything working properly, but you're not sure why that box had to be unchecked? Or is something still amiss? Please describe.

I recently updated to 91.5.1 (64-bit) on Windows 10. I usually keep up-to-date. I never had to reset the color after an update before. I'm trying to understand what the "reader's colors are" and why, even though the faded colors that appear when the "Use reader's default colors" box is checked are the same as the colors shown when the box was is checked, the background color is not the one I want unless the box is unchecked. This appears to be a bug introduced in the release. If it was intentional, I'd like to know the reason.

The confusion, I think, is in TB's desire to be flexible. The option is that, when checked, it shows the default system colors. When unchecked, it shows your preferred colors. The puzzlement is that this option is in the Composition window, spelled as 'Use Reader's Default Colors', and this choice is ALSO in the General section as "Use System Colors' - one affects the compose window and the other reflects viewing other messages.

Ok but in the Composition Window the desired background color is shown when the Use-reader-default-colors box is checked. But the background in a new message is white. Surely this is a bug. Either the reader defaults are not used when the box is checked or the wrong color is displayed. When the box is unchecked, the same background color appears and that is the color used in new messages.

re :The with latest release, new messages started showing up with a white background. To fix this I had to go to Tools>>Preferences>>Composition and then under HTML style I had to uncheck "Use reader's default colors."

That is illogical. Preferences>>Composition and then under HTML style The section you are talking about is purely to do with the 'Write' window and composing emails. It has nothing to do with emails you receive. Selecting the checkbox: "Use reader's default colors'" means use the same default for creating emails as is set up to be used for emails you receive. The next part describes where that is set up. So if you want to read new incoming emails AND compose emails with a pale yellow background, then select the checkbox: "Use reader's default colors, then follow instructions below.

For emails you receive and see in the 'Message Pane' the settings are here: Preferences > General and then under 'Language & Appearance' Click on 'Colours' Text = Black Background = pale yellow (if that is what you like to see) Do not select 'Use system colours' - leave unchecked. Override the colours.... set to 'Never' click on OK

It means all incoming mail that does not have a background html css settings in the email will use the pale yellow choice you selected. This means all plain text, simple html and html that does not have a background precoded will show pale yellow.