You can create a unifying theme if your chosen themes accommodate one another. Look at this: your spouse wants an Asian theme, your teenage daughter wants a gothic theme and you want a tropical theme. In such varied combination you may decide to do an around-the-world theme with your living room; this is like a miniature version of Disney's Epcot. You may incorporate Asian tropic samurai, plants and dragons swords that would incorporate all three themes. The point is, regardless of the themes, there is common ground can be found.