Apple Mail app for Monterey 12.6.3 will NOT let me darken the background for creating or reading emails with "light background" checked in System Preferences /General. I don't want to use dark setting. Solution?

When in the Dark mode appearance, Apple Mail gives you a selectable settings choice of a dark or light viewing window for messages. That also sets a light background for composing messages. In the Light appearance mode, everything has a light background in Mail and the first sentence options for light or dark are not present.

The dark setting is a global setting in System Preferences. Mall adopts whatever the global setting is, and only impacts how you view the message on your computer. Just go back to the System Preferences and choose light mode if that's how you want to view it.

Apple menu -> System Preferences -> Displays -> Color helps you modify the spectrum used for the warmth of the color of the display in general.

Mail does make any adjustments to the actual message itself when it comes to the message background as sent to other users. That's controlled by your attachment sent.

If the user what's to dependent on time of Sunrise and Sunset ?

Explore the Auto feature in that same area of System Preferences earlier mentioned