My favorite wedding that I have ever been to was a fair wedding. Complete with bales of hay in the shape of their initials, mason jars for drinking, a wedding pie instead of cake and kettle corn between the ceremony and reception and a fireworks finale. Amazing wedding!!!
I know a couple who did a medievel theme. Both wore custom-made outfits - hers was a gorgeous gown in burgundy velvet. I know someone else who did "Star Trek".
Some lovely ones I've seen are Circus (everyone wears costumes), Fairytale (very formal), Western (casual), Rock N Roll (slightly formal with fun ties and entrance music).
Some ideas for wedding theme are "tea for two", fairy tales theme, and depending on what season you could use snowflakes, daisies, a beach theme, or "fall in love."
I have done some work as a Wedding Planner, and have seen some wonderful theme ideas! Some of my favorite are, halloween, time period, pirate, or folk festival.
Many people use themes that correspond with the time of the year -- fall, Valentine's, America (4th of July), etc. You could also use a theme of something that is important to you -- seashells if you are from the beach or something from your college or even something from your career.
The most elegant 'themed' weddings I've seen: a Renaissance wedding (horses, costumes, outdoors), a relaxed 70's style(not hippy), and a New Years Eve wedding. Remember- Keep LOVE as main theme!
Most people go with the season as far as picking colors and such. If you have it in the spring then you would have more pastels and you could have it outdoors with flowers. If you had it in the fall leaf changes and natural colors like red, orange that mimic the leaf changes.