The favourite candidate for the missing Theme Word appeared to be BLACK, supported by Woodside187's interesting suggestion of it being endorsed by the answer ARMBAND. What puzzles me is that the only unclued entry that had no alternatives was NIGHT (N-G-T), and I cannot see how BLACK can be the Theme Word as neither BLACK NIGHT nor NIGHT BLACK seem to mean anything.
Has the trail gone completely cold, or has someone actually worked it out?