I'm another one who completed the grid without knowing what it was all about, and that took me till Monday morning. Now, 2 days later, Google has finally found me the theme, but only by going to about the 5th page of results! So now I realise what I should have Googled straight off. Obviously I don't know, but it seems to me that the editors may have decided that if you did not actually know the source, finding it was too difficult, especially without the Internet.
Anyway it does all come together very neatly if you persevere.