You may 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 choose to do an around-the-world theme with your living room; this is sort of a miniature version of Disney's Epcot. You can incorporate Asian tropic samurai, dragons and plants swords that would incorporate all three themes. The thing is, regardless of the themes, there may be common ground that can be found.