Whatsapp theme suddenly turned dark I was chatting on whatsapp, then I didn't 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 it back to previous theme, even if I select default theme or light theme etc. 
How to return to previous theme? I was using default. I had not changed any setting for theme or Wallpaper 

I use latest version of android (I guess) , and my phone is Samsung Galaxy S9
As suggested in comments, I looked at my phone settings and it was in dark setting. Whatsapp likely upgraded in the background to default theme so it automatically turned dark (latest version of WhatsApp does that).

To revert to light theme, from chat settings, change it to light theme. If it doesn't change, clearing cache and rebooting may help