Grand Canyon is a very strange bird. It's a completely unique urban piece, where relating the entire plot would fail to convey much.
It's central theme seems to be the inherent uncertainty life holds for people of every race, background and station. But to proclaim that THE theme of the film would be to horribly understate its scope. Similarly, to pigeonhole it in a particular genre is futile.
The film has volumes to say, though likely different volumes for every viewer, and says it all in such a non-preachy way from so many angles, that in the end, i can't even define its central message for myself.
Nevertheless, it does it's business with such laser precision; every prop, line of dialog, and bar of background music contributing to it's pervasive mood and powerful message, that i'm pleasantly surprised, and come away very thoughtful after every viewing. Still it doesn't feel at all stuffy. A sparkling film with a great cast and everything working.