Hi
@fahad26A,
Welcome to the Android Central Forums!
I don't know of a dark mode on the Galaxy Watch 6. I've had my Watch 6 Classic for almost a year & I've never seen it. (But maybe someone will know.)
A couple of thoughts...
~If your Watch's power saving mode is on, the screen will be dimmer.
~If your Watch's sleep mode is on, the screen will be dimmer.
~If you're looking for Samsungs 'Dim Mode', it was only available on the Tizen OS models not the WearOS models.
~ They only other thing I can think of is the inversion setting in the Visibility enhancements, but, I don't think that's what you are looking for. I attached pics from my watch.
View attachment 364479
View attachment 364480