Plain text e-mails in Dark Mode Mail 12

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?

There is a Terminal command to generally prefer plain text E-Mails over rich format. However, I haven't tested it under Catalina.

defaults write com.apple.mail PreferPlainText -bool TRUE