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