IF you're bike is highly decorated, 90% chance it will not get stolen.
IF you're bike is some $20 craigslist mountain bike with absolutely nothing changed, 50% chance it will get stolen without a lock.
IF you're bike is locked, you are 100% safe.
IF you do not worry at ALL about your bike being stolen, you are 100% safe, locked or unlocked, decorated or not.
I've gone two years so far without locking my bike. It's a plain vintage 70's women's 3-speed dutch style bicycle I bought for $25. No decorations, no fancy lights, just a front basket, a blinky light, and a mount for an umbrella. I even leave my backpack in the basket with random stuff I need at night. It's never gotten stolen. I've left it overnight by an art piece in the middle of the playa overnight, and gotten it the next afternoon. I never worried about it, ever.
My brother on the other hand, has a bike pretty much identical to mine, and he worries like mad. His bike's been stolen once, and his backpack stolen once each of the two years.
So don't worry about it and enjoy yourself.