IF THE GOYIM DO TURN ON THE LIGHTS ON THEIR OWN CAN THE SIMCH PARTY DERIEVE PLEASURE FROM THE LIGHTS?
If there wasn’t any light on in the Simcha Hall and the people would not be able to eat without the light that was turned on, you would not be allowed to derive benefit from the light since the non-Jew turned it on for the benefit of a Jew. However, if the light that was turned on merely added to existing light, you would be allowed to benefit from it.
All the best
Shulchan Arruch OC 276:1 and MIshna Berura 13, Shulcan Aruch OC 276:4