macOS Mail: How to remove background color of a message in message list? In MacOS Mail, we can change the background color of a message in the message list.  This could happen in two ways AFAIK: (i) Select a message, go to Format -> Show Colors, then choose a color, or (ii) Setup a rule that automatically set the background color based on some conditions.

The question is: once the background color is set, I can't seem to find a way to remove the background color.  I can only change it to another color.  This does not work well when we switch between dark mode and light mode. What I would like to do is to remove the colors all together (or equivalently, make the background transparent).  Is there a way to do this?
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.