Hey there. Are you talking about the contents of an email, to the right of the window as shown in the image?
If so, you're unable to change this. If I'm assuming correctly most emails contain HTML content which allows us to format the email with custom images, colors, links, buttons, etc. This means that Apple (or any mail provider) does not have control over the background of the email contents. They do have control over the background that is below the email. If we use the example shown above we can picture that if you lifted the email up, below it would have the window color around the email's contents. (Dark brown/yellow transparent color)