Anki cards in day mode looks ok:

…but are unreadable in night mode:

The only workaround for all cards is to force white background in night mode:

…but this remains a broken night mode.

Looking at individual cards source, we can see that colors are declared directly in them:

It would be much better to have styling in one place (styling), then add dedicated .nightMode colors there.

I cannot find flashcards sources in this repository, so I don't know how provide a patch for that.