I have made a custom theme on big cartel, and everything is perfect, except one thing. I would like to have the cart update without going to the cart page when adding an item to your cart. I have made my custom template over the "sexy" theme and have no idea how I go about implanting this
I know this can be done, because default themes like "Good Vibes" do this.
You'll want to take advantage of the javascript API: https://help.bigcartel.com/developers/themes/#javascript-api
With this, you can drop in the line of code to load the API into your theme and have access to add, update, and remove items from the cart using javascript.