what is candlelight dinner
Although this is splitting hairs, all I can think of is: A candlelight dinner is a dinner by which the lighting is derived from lit candles. I think one always assumes a dinner involving candles usually has the candles as the only source of light. So, in my mind either would be correct. However, I think I have usually heard them said as follows: They ate their dinner by candlelight. (Candlelight is a noun)