Setting the idles and fuel/air ratio

Headlights should not affect idle that much. Biggest effect on idle speed after obtaining a proper "curb idle" is if you have an auto, in drive.

My Dad, rest his soul, was one of those people who always wanted the slowest idle possible. His always had his old Chivvy pu with straigh 6 idling at about dead slow. I'm the opposite. I don't mind if the idle is a little high. My little 318, with a really mild cam, used factory (low stall) converter idles at 8-900 in neutral, and that's the way I like it.