Admin notice in unreadable colors

That light green comes from:

#message .jp-banner__content {
    color: #dfffcc;
}

BTW, it only gets green in the first screen. It turns black if I change screen without touching the notice options.

The second option, which, btw, is the one that applies to this situation – “Ignore the difference. This is just a staging site…” –, doesn’t work…

Yes, that’s indeed a known issue. It will be fixed in the next Jetpack update, scheduled to be released this week.