Since the comments to my question indicate no one else was seeing any changes, I concluded that the problem was something I had done recently to my system settings. And, indeed, it turns out that was so.
I will explain in detail so others can avoid the problem I created.
I have vision problems and they are getting worse as I grow older. On of the problems is that I have difficulty seeing fine detail in low-contrast situations. In particular, I have difficulty with text that appears against a colored background. Recently I searched on the web for advice on what I might do to improve the visibility of such text. One suggestion I found was to adjust the Display contrast slider in the Accessibility panel of macOS's System Preferences. I tried this and it seemed to work pretty good.
For example, at the normal contrast setting I can't see the musical note that appears on Music folder in my home folder, but I bet most or all of you can see it.
But I can make it out at a higher contrast setting.
Unfortunately, when I visit this site with the higher contrast setting, code looks like this:
The gray background is completely washed out. Background color is similarly washed-out for code results and quoted text.
I didn't make the connection between that "new" look and the contrast adjustment I had made earlier, which is why I posted the above question.
I now face making the decision on whether or not to go back to default contrast and loose the better visibility of text and icons it gives me in many situations or keep the higher setting and accept loss of discrimination of code and other markdown blocks on this site.
Sorry to bother people here with what turned out to be a rather stupid question, but the incredulous responses really helped me to resolve the issue, so thanks.