Uh ... why? This is your top priority?
Yeah, it's just incredible that I'd think darkness is important in a first-person computer game centered on surviving Lovecraftian horrors, isn't it? It was my top priority because tailoring one's game so that consoletards 1.) can always see and 2.) never get lost in your simplistic level designs is a key marker of the decline.
Because darkness is scary and Cthulhu is also scary I guess? Darkness isn't a theme in
the original story at all. There's the big portal of darkness Cthulhu comes out of at the end that has 3 seconds of camera time, and a couple of oblique mentions of darkness like "Cthulhu lives in a dark house somewhere" or "the aliens came from a dark star eons ago" and that is it. There's no stumbling around in dark caverns, no searching through creepy old mansions by fleeting candlelight.
I've read every single story written by Lovecraft at least twice over, and many several times each.
Measuring the amount of darkness present or mentioned in that specific story—most of which is narration and exposition that takes place in various everyday, well-lit buildings—is ridiculous. The game we're discussing doesn't take place in mundane locales or upon eldritch R'lyeh risen from the bottom of the ocean, and seems to be much more akin to The Shadow over Innsmouth.
The search for the cultists in the woods mention a "black morass", implying black swampy ground, but the entire sequence is filled with vivid visual descriptions of what everybody saw. The forest would have been dark except it was lit up by a red glare at all times. There are zero instances of any character described as having a hard time seeing anything. Except for the forest scene everything happens in broad daylight. There actually is way more talk about the color green.
Aside from the fact that that one story is only a tiny fraction of the entire Lovecraftian mythos, there's a big difference between a book that relies on description and imagery and a first-person immersive computer game that actually visually places you within the scene. Also, there's no need to beat readers over the head telling them when it's dark every single time it happens to be dark in a story. Rest assured, in many of Lovecraft's stories, during "action sequences" (i.e. not investigations or conversations in a mundane location), there is plenty of darkness to contend with; and I think that a subterranean tunnel qualifies.
Why do you think Lovecraft specifically mentions the red glare when it's nighttime in the woods? Readers will rightly assume that it's pitch-black dark at night in the woods, or nearly so, unless you tell them that either the moon or some other phenomenon is causing it to be otherwise.
Yes, there is also often some form of luminescence or other atmospheric presence in the "action sequences" of Lovecraft's stories; and obviously, protagonists usually have some means of combating the darkness even when it is dark. Do you really think that luminescence a la The Colour Out of Space is the reason why we won't find one bit of actual darkness in this game, though? Come on, now.