A really simple way is to use a 120v SPDT relay. Wire your indoor lights to the C and NC contacts and connect the coil of the relay to the output of the Lightning controller (where your "lightning" lights plug in) When you get lightning outside, your indoor lights will flicker off, it's not really a brown out so to speak but it does give a great effect. If you really want a true "brown out" effect, just wire a parallel dimmer to the C and NO contacts, when the relay energizes it will cut the full power side of the circuit and re-direct power to the dimmer.