When using the "Night" site interface (the dark-mode skin), the "tour" popup labeled "Diverging changes are shown line by line." contains black-on-dark-gray text below that initial heading line (which is rendered in white). No idea if any other interface elements are similarly affected.
I'll also report this over at Reading/Web/Accessibility_for_reading/Reporting/en.wikipedia.org, but presumably it isn't limited to enwiki at all. It's also one of those rare dark-mode issues that can't be fixed with a content tweak, since it's actually an interface bug.