That all depends on how you are turning on the lights. If you have used "Screen Color Tone" to make the room dark, then just call a common event from your skill, and set the color tone back to (0, 0, 0, 0). If the player uses the skill in a normal room, it won't matter, he will just waste SP.
If you have used a dark picture or fog, same deal, use the common event called from the skill to turn off the picture or fog.
There is no way to check in a conditional branch to see if the skill "has been used". You would need to set a switch or variable one way or another to indicate that the skill had been used. then check the variable or switch in your conditional branch.
If you want to leave the lights on in that room even after the player leaves & comes back, you'll need a switch for each room.