Candlestick, I got very bored searching the internet for sufficient details to complete the puzzle, and that's even after I had the relevant title. What led me to the theme was to apply the second part of the instruction from the penultimate paragraph of the preamble. There's a limited number of options, and one of them is quite revealing.
I've no idea how to resolve the dilemma about the code options. Since the source material is not available on the internet it's not possible to discover what the thematic error referred to in internet synopses might be.
Regarding the line above the grid, my solution agrees with djawhufc's proposal. I think it's pretty daft but I cannot think what else to draw.
Overall I think the solver is expected to engage in far too much manipulation and mutilation. It's not what I consider an elegant puzzle, worthy though the attempt is. The puzzle would have benefited from a restraining hand.