I regularly deal with e-mails from a company using SalesForce where the messages are sent with a bright white background for the entire message body, set in the HTML.
In Dark Mode this is exceptionally uncomfortable, as you'll be ticking through your e-mails reading white on black, when suddenly you're hit with an almost full screen of white.
I checked the raw message contents and I do see that there are two parts, an HTML version and a plain-text version. However, I cannot find the configuration option to specify that I want Mail to render the plain text version of the message. So how can I do this, or otherwise override the colours of these messages?