Hey there, Debr. 

La Diosa has already given an excellent description of the Bohemian lifestyle. Thanx for that, LD!    The only thing that I can think to add is that their style is somewhere between hippie and gypsy.

As for goths, they are people who belong to the gothic subculture, which emerged in the late 1970's, quite closely to the punk rock scene. For this reason, I very much doubt that your 15-year-old neice is a goth. Most true goths in the US are over the age of 30 (thus, even I am young by that standard).
    That being said, I don't personally know your neice, so I can't say for sure one way or the other. Her fashion style sounds gothic, from the way you've described it. But again, that doesn't mean that she is.

You also don't have to capitalize the word "goth" when referring to the subculture or its people. It's not required. 

Anyway, as far as gifts go, goths would also appreciate something "artsy," so long as it had a darker æsthetic. Music, jewelry, artwork, clothing, giftcards...the list goes on. Some of my favorite gifts of the past have been black satin pajamas, 1928 jewelry, giftcards to my favorite clothing stores...or Starbucks    and lots of CD's. My family has also given me calendars of the world's most haunted places as well as tons of Haunted Mansion and Nightmare Before Christmas stuff. 
    You already know some of the things that she likes - work with that.

If you can't decide that way, then you could just get her something that will come in handy or useful in daily life. My parents will usually give me a gas card or Target giftcards as a bonus Christmas or b-day present. Oh! And movie theatre gift certificates are great too!

Hope I've given you at least a few ideas. Do let us know what you get for her.