first and foremost, congratulations!
and secondly who gives a stuff what your friend thinks. I'm sure she's a wonderful lass BUT it is your wedding and this is the day when you do the things you want to please YOURSELF and not everyone else. if its your idea and you have it flow throughout, then it's themed. there is more beauty in the random than the repetitive. especialy if it's sentimental. it makes every table, every corner that little bit more intriguing.
sure you can make life easy by picking the few definite colours you want to theme with and stick to them, and then go from there. also if you still feel like it's all a bit far out and want to hone it in a little then go grab some ideas from decorators and party themers. (Ive just started an events management course and my eyes have been opened to these BEAUTIFUL people)
Im in oz, but I have a couple of american sites that we got in a media pack from class.http://www.bizbash.com/http://www.party411.com/http://www.sydneyparty.com.au/index.php?topic_id=14
(this is a sydney based site but you can check out the theme companies on the right side of the screen check out galleries and get more ideas)
then your only problem is deciding when to stop and decide!
I wish you all the best, and remember, the more sentimental the more meaningful and memorable it will be for everyone