It's because people should be using the "automatic" color option rather than using straight black (or white). This usually happens when people paste in text from somewhere else which was already colored black, but then don't change it back to automatic (probably because it looks fine on the lighter colored theme(s) that they're using), and, as far as I know, I am the only person who goes around changing this wherever I see it because I use one of the dark themes and it bothers me to no end.