well the main theme that carries through all of lord of the rings and silmarillion is the battle of good vs. evil, and at what cost and sacrafice it takes to defeat evil. plus i really like the idea of no matter how hopeless the cause, it is worth it, i.e. frodo destroying the ring, the noldor in beleriand, beren and luthien ........
If you have't done your report yet, I strongly suggest you read the preface to the book (2nd edition,) which suggest there are no real "themes" and certaintly no allegory to actual current events (at the time) or history, at least it was strongly his intention, as he says, that it would not be that way.