Mails I receive from Discourse look like this since about a week or so. Possibly because I use a dark theme on Discourse, but my Outlook is not?
I haven’t seen any such thing on my Safari, so I would think something must have changed on your end. Any browser or OS updates? Any browser setting you may have inadvertently changed?
This is not in any browser. I am receiving mails in Outlook that look like the attached screen clip.