I'm testing using Vivaldi as my email client again. I figured out that I can go to the Vivaldi settings and under Appearance I can check "Force a dark theme on all web sites." This has the desired effect of rendering HTML email as dark mode. It also alters all the sites you visit appear to have a dark mode. So far, so good with that. I don't think you can turn it off site by site though. We'll have to see if anything "breaks". For now though, I'm not greeted with a flash bang of white background when I process my email so that's a win.
#vivaldi