How can I change background color in Mail?

Think you need to Go >> System Setting >> Appearance

At the Top of image are 3 Options >> Light, Dark and Auto

I have it set to Light

Then changed to Dark and presto the Background in Mail changes to Dark

If you Set it to Auto, it should pickup the Time the of Sun Rise and Sun Set in you location and Auto change for Light to Dark

Use dark backgrounds for messages

Excerpt from above Link

Show messages in the preview area and new messages you’re writing with either a dark background (select the checkbox) or a light background (deselect the checkbox). This option is available only if you set Appearance to Dark in Appearance settings.

When this checkbox is selected, you can switch to a light background while viewing or writing a message. Choose View > Message > Show with Light Background. To show the message with a dark background again, choose View > Message > Show with Dark Background.

thanks apple for burning my eyes!!!!

ridicluous.

Sorry I made an error in previous - that is not for Mail but System Wide

Choose View > Message > Show with Light Background. To show the message with a dark background again, choose View > Message > Show with Dark Background.

Actually, it was my learned colleague 👍 @ Luis Sequeira1 who had the Best Illustration to follow.

Having moved from " 12 year old MacBook! " to where you find yourself now is a major move.

Do come back to these Forums if or when additional question may arise

You can choose View->Message->Show with Dark Background (or "Light Background" as the case may be).

FWIW, from Monterey…

skybunkerboy wrote:

also can the email background color of the mail document actually be changed as in previous versions?

No. At least I can't.

Just to add, Monterey was the last version of macOS that one could actually assign different colors to the background in Mail. If you want that option, you need to use a different email client.