Ok, since FF6 every FF excpet 11 had a theme, and I think they are very good and support the games well. Many themes could still be used, like Passion, Good And Evil, Power, Freindship, Sorrow, etc. Now, here they are!

FF6-War
FF7-Life
FF8-Love
FF9-No Clue!
FF10-Faith
FF12-Freedom

If you know FF9's theme tell me.