In D&D 5E, dim light condition only affects perception (wisdom) checks. If I interpret the Player’s Handbook correctly, for combat rolls,
- creatures that don’t have Darkvision, whose target is in dim light, and
- creatures that have Darkvision, whose target is in dim light or in darkness (within range, so 60 feet for Elves, for instance),
have no disadvantage on attack rolls.
This makes it difficult to use darkness and dim light in combat. You could argue that in a poorly lit dungeon, it would be more difficult to succeed an attack roll, especially in ranged attacks. But the ruleset does not provide a way to do it with a disadvantaged roll, as stated above, so I’m trying to find an alternative way to use light and darkness as a strategic asset.
Considering this, here is my question: Would it be correct for the DM to interpret dim light as cover, if they wanted to use light and darkness as strategic asset for enemies when designing a particular map, such as a dungeon, ruins, tunnels, …? Or, if not strictly correct from a ruleset point of view, woult it be acceptable as "house-rule" (when taking care to warn the players about it, of course)?
If not, is there an alternative way to use light in combat strategy, considering a party may have many creatures with Darkvision?