You may create a unifying theme if your chosen themes accommodate one another. Think about 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 in your living room; this is sort of a miniature version of Disney's Epcot. You can incorporate Asian tropic dragons, plants and samurai swords that would incorporate all three themes. The thing is, regardless of the themes, there may be common ground to be found.