You can tell from the tooltips when your characters have Disadvantage to attack because of darkness, or "target obscured". Then you need a light source or Darkvision.
I think they changed many places from Early Access to not be dark anymore in full release. Which is understandable because those places weren't actually visually dark enough and you could see just fine. Getting a penalty was unintuitive.
Personally I think many parts of places like the Underdark should be darker, and areas shouldn't be so evenly lit. Darkvision and light sources should come into play more in combat and there should be a bigger contrast between darkness and light. Monsters who have advantage in the dark should make the most of it.
