On Android 10 the latest WhatsApp will adopt a dark theme is you select that (WhatsApp Settings > Chats > Theme) or if you select "system default" and your system theme is dark. But on my Pixel (Android 10, March security patch) selecting the "light" theme in WhatsApp settings will turn it light, so the question is why doesn't that work on your S9?
So, questions:
1) Does your Samsung change theme with time of day? Just asking because you said that it changed colour during a WhatsApp session, and that's the simplest explanation for that (the app following the system). But that isn't yet a standard Android feature, so if your phone can do that it's a Samsung addition. The reason I'm asking is that if this is a manufacturer add-on it might interact differently with the WhatsApp app from the standard Android 10 theme selection (just a guess, but I'm looking for a reason the "light" theme might not work on your S9). What happens if you change system theme settings?
2) You say it turned dark "with a wallpaper": what did you mean by that? WhatsApp chats by default have a wallpaper, though in light mode you might not have noticed it: it's a bit more visible in the dark theme. If that's all it is (a dark version of the standard) then all that's changed is the dark theme. If it's anything else then this might be pertinent to working out what's changed. If you select the Wallpaper in the "Chat" settings and turn it off does it go away (then you get a plain background in chats)?
