whatsapp theme suddenly turned dark

I was chatting on whatsapp, then i didnt look at my phone for 5 seconds, and when i looked back I saw that it turned dark theme with a wall paper

Why?
And now I can not get ti back to previous even if I select default theme or light theme etc.... How to return to previous? I was using default I had not done anything special for theme or wallaper

I use latest version of android I guess, and my phone is samsung galaxy s9