Black Background after core update to Drupal 8.4

After I updated Drupal core to version 8.4, I attempted to create a new slide show. I was successful in creating the slideshow; however, for this particular slide show, I needed to display the slide title. When I attempted that feat I discovered that the title was in blue on a black background - virtually unreadable. The blue was OK - defined by the theme, but the black was new. I went back and checked a similar slideshow that had been generated before the update and the background was a light blue - again as specified by the theme. I then went back to a copy of my site before the update and created a new slideshow. Again, the background was correct. At some point, I investigated and found that the black background is for the slides, not just the title. Inspecting with Chrome showed some HTML that specified rgb(0, 0, 0).

Is there anything that I can do to get a usable background back?

I am not pointing to a website because I can correct the immediate problem by modifying an existing slideshow, but I have some new sites coming up where this will bee a problem.