Hello.
Using Quark-Ecommerce-quickstart I discovered that 'Add to Cart' button doesn't work properly for variable product. Debugger doesn't show any JS errors/conflicts. The thing is that in your demo, when you choose to discover a variable product the 'Add to Cart' button and the quality selectbox nicely appear when user selects the option of the product. In a quick start installation, 'Add to Cart' button and quantity option is always shown. Some CSS missing I believe. Can you please check that?