Theme hints: Today’s puzzle’s theme works differently from most. This time, the title’s meaning becomes obvious mostly after finding the “trick” that holds the puzzle together, so it isn’t a big help in discovering what’s happening. The unusual shape of the puzzle is meaningful. The circles in the squares are, like the title, a nice touch, but unlikely to “unlock” the theme for most solvers. The other thing that’s unusual is that the long answers, like the title and the circles are more of an afterthought, a nice tie-in to the theme, but not crucial to solving the puzzle. Without giving the answer away completely, my suggestion is to look carefully at 106-Down and 103-Across. Once you figure out what’s going on there, the theme will reveal itself to you in a way that will be a big help in solving the rest of the puzzle.
A few specific non-theme hints:
6-Across Fixes keys: “Fixes” is a little misleading. Think piano.
18-Across Brother of Prometheus: While poor Prometheus was chained to his rock, his brother was certainly holding up his share of the bargain!
45-Across Bee product: Forget about insects or spelling contests, there’s another kind of bee bee-ing clued here.
109-Down Corner piece: Think game, not furniture
65-Down Tries: 4 words
67-Down Pressing needs?: Think gym, not laundry
69-Down Unlike eagles: Think sports; Is this the opposite of how you feel after you ate too much Xmas dinner?
73-Down Next-t0-last Beatles hit: They followed their own instructions (the title of this song) shortly thereafter.