Since you use the dark theme, the fix is easy. Just install the addon, Dark Reader, and it changes background to black and text to white,, for both reading and writing. There may be a few incoming HTML messages that insist on their colors, but I've used Dark Reader for yeas and like it a lot. You can se this at settings>general and scroll down to Fonts&colors and click the colors button, You can there set the background color. And be sure to NOT tick the 'use systems colors' box. Like you, I don't like white and the setting is simple and works for both reading mail and in composing mail. We vets gotta' stick together. Thank you for your service.

Thanks David for letting me know about this add-on.. I installed it and it works on about 95% of the emails. But that is better than zero.

-Scot p.s. thanks you for your services as well.