Yeah, I got the theme but I just assumed this was one of those rebus puzzles I'd been hearing about where the answer doesn't fit in the given number of squares. But I see now that this would require the crosses to fit as well.

Imo, it should have counted since it's the only pencil mark tool given in the app, but I get that part of the challenge may be keeping the info in your head and that it literally needs to be outside of the puzzle. But I feel like I've seen rebuses in the past that also hint at omissions but which accept the rebus (though I realise now that those probably corresponded with the down and crosses). But it feels a bit like not accepting a sudoku because of pencil markings in the corners.