We currently have the ability to upload your own theme and customize existing themes, in development. Shoot me an email, chris (at) weebly.com, and I'll give you access. Otherwise, this feature should be live early next week =)
Weebly is a great concept, but right now they only have a small selection of relatively bland templates... Until they open it up to outside custom templates (or find a way to convert wordpress templates!) it's a hard sell.