how to change the default background color of the body of an existing email

so this is cross posted here:

the background color of an existing email is white. i want to change that to a gray color. i have searched the internet but have not found a solution.

using dark mode to change the color of menu's and sidebars is working, just have to get the white to a gray.

can this be done?

can this be done or should i abandon the thread?