(first image) Here is what happens when the author of an email uses some basic tools such as 'background color' and you open the message in Mail.
Solution: Mail Prefs->Viewing: uncheck "Use dark backgrounds for messages"
(second image) Again in Mail, SpamSieve dumps stuff into the "Spam" mailbox. For some unknown reason, Mail displays the list of messages differently making it impossible to read the title, date, etc. without actually selecting the message. The dark text on the dark background seems to be the worst possible combination. Of course, white text on a nearly white background runs an extremely close second. I wonder if that's what happens in the "Undark" mode?!