Inverse Main Frame with specific firefox client

I have nextcloud 20.0.11 installed. With one web Client (Opensuse 15.2 KDE 5.18; ; Firefox 78.13.0esr (64-Bit)) i find an inverse Main Window. While at all other systems I see white background and dark grey text, this is inverse at the specific PC.

So far I coud not find any explanation for this behaviour. May be anybody has a suggestion about the cause?

Thank you!

This is the odd behaviour:

normal view as followed: