Page background black with black text, cannot read without highlighting.

For some reason, several different pages appear with black backgrounds and have black text. I tried disabling dark mode, but the issue persists. An example (YouTube Studio) is shown in attached images. I'm using it because it's a well known site, and hopefully, the way the page is made can explain why this doesn't happen on every site.

I don't have any add-ons that would change page appearance, and I tried with and without Firefox's built-in dark mode.