macOS Mail: How to remove background color of a message in message list?
I know a year old, but for others:


Open a new email. 
Paste a screenshot into the new email.
Select markup in the top right of the screenshot image.
Select a color in the markup tool and open the colours dialogue. 
Choose any color, but reduce the opaque to 0%
Drag this 0% opaque color (transparent) to the preset slots on the right of the color dialogue.
Close the dialogue and the discard the email.
Make a rule in Mail that is trigged for all emails. 
Set the background color in that rule and select the preset with the transparent colour.
Select all your emails, right click and let rules run for these emails.


Have fun.