Such nonsense. The BG2 beginning is already super dark. BG had his own style. Sure it's not always super dark, but it's dirty and everywhere can be some maniacs or psychopaths and there are Cleary some really dark parts like the beginning.
"release master, I no longer wish to come back"
Bullshit.
Just because it starts in a dungeon and you throw in the words "torture" and "trauma" it doesn't give a particularly dark tone to the entire adventure.
If that was the case Divinity Original Sin 2 would already be orders of magnitude darker than BG2 ever was.
BG2 gives the impression, that is way darker, than DAO2, as it has a less cartoonish art style.
In DAO2 everything is on fire all the time, all ability animation are pretty colorful, if you swing your sword
it is guaranteed to make you shine.
The torture chambers, are sometimes dark, but there is always a funny zombie, burning sheep, a cheese joke,
that kills the "dark" tone. Which I think it is good, because it gives its own unique identity,
but not for Baldur s Gate game.
There is also no night time in DAO2, and some of the areas, are bright as the sun itself.
BG2 does not have any areas, that are very-very bright.