If your chosen themes accommodate one another, you can create a unifying theme. Consider 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 like a miniature version of Disney's Epcot. You could incorporate Asian tropic plants, dragons and samurai swords that would incorporate all three themes. The point is, regardless of the themes, there is common ground to be found.
Even if you inherited from Aunt Kay and you aren't particularly keen on the style you can invest in slipcovers for the upholstered pieces in a neutral fabric to unify the pieces, then use your creatively through accents.
Having said that if you do have the money to purchase some new furniture then that is a lot more fun. It is important to set a budget though prior to looking as you know what it is like when you are out shopping and you get carried away with what you see.
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