Normally when you hover over an enemy to target them, it’ll tell you if you have disadvantage and include the tag “Too Dark” if darkness is an issue. At least for me. I’ve only seen that in Act 2 though.
More often my characters miss when their primary stat isn’t high enough, or if they’re casting a spell not using their primary stat (e.g., a cleric using a scroll, which relies on Intelligence).
My Lae’zel missed pretty often when her strength stat was 17-19, but once she hit 21 (… thanks Astarion), she became an absolute beast and rarely missed. So you might want to focus on bumping up characters’ primary stat.
If you’re worried, light is a common cantrip so it’s easily accessible. Because it’s a cantrip, you can cast it as often as you want while not using a spell slot. But the only time I worry about darkness is Act 2.
