We're changing the extremely old fixtures in our hallway with some sleek modern LED lighting.
Upon removing the fixture I discovered three sets of twisted wires - double white, black-red (the fixture used it for the black) and a black-white pair that wasn't connected to anything. The electrical box itself is grounded (I think - there's a bare copper wire tied to a screw in the electrical box) but the new fixture says to attach the green fixture ground wire to the "power supply ground wire" which, as far as I can tell, doesn't exist.
The previous fixture had a bracket with a grounding screw but that bracket is incompatible with the new fixture, which came with it's own plate/bracket system (but nowhere to put a grounding screw).
How do I properly ground the fixture? Would the untouched black-white pair possibly be a part of the solution?