The black-on-black settings appears to be an issue with some WinJS styling. We're looking into it.
Thank you, I appreciate it muchly.
Are you saying that the syntax styling also doesn't work in High Contrast Black, or are you saying it isn't working at all on your system regardless of theme?
Simply Text 188.8.131.52; Lumia 950XL, Windows 10.0.14965.1001
With High Contrast Black, all document text is white on black
With High Contrast Light, all document text is black on white
With High Contrast OFF, document syntax style colors work
The High Contrast theme is, as I understand it, meant to improve readability by removing 'decorative' colors. Syntax colors are 'functional' though, and I think there is a way to declare styles as 'important', 'required' or similar, to make them work with High Contrast themes.
-ms-high-contrast media feature
High contrast themes