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.
Because you love them individually, they will work when you use them together and achieve an inviting and comfortable feel to your room.
If you already have lamps then you can just change the lamp shades and this can affect the lighting. Lamp shades are a really cost effective way of doing things. Just keep it simple and experiment with different options.
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