I can only speak from experience.
Caves, like Tom’s are dark to human eyes, but rocks and the ground tend to be a bit warmer than the air, and this provides infrared light that demons can see just fine by. Same is true in Doom, and now that it’s started, there is plenty of infrared light to see by, it was rather dark when we got there, which is why the D’Orcs had fires and torches and glowing rocks.
And now, the actual lighting system is back on/available.
The sky emits a broad spectrum of visible light that tints red and as far as I can tell is perpendicular to the ground acting pretty much like your sun (which comes in perpendicular given the distance)
If one assumes the Abyss is infinitely flat, than this light plane must be infinitely flat as well. In terms of distance vision, since all the light is perpendicular to the ground, then as you look to the distance, the light over that region would appear to be at an angle to you, much like when your sun is overhead and you look towards your horizon, however, while similar, it is different because there is no corresponding curvature. So it’s less pronounced.
Now, that being said, there are flashes of light from flames and plasma balls that also affect vision, or at least cause ones pupils to sometimes have to adjust. If the burst is close to you, it can sometimes temporarily blind you if you aren’t careful. This is a problem for newbies.