Why can only one website template be active, and how do you change the active one?

Only one template is active at a time because the storefront must present a single coherent experience — two live themes would be ambiguous. You can install multiple templates and change the active one via the activate action in the Templates area, which switches the storefront to the selected template. So changing your store's look is simply activating a different installed template;…

Only one template is active at a time because the storefront must present a single coherent experience — two live themes would be ambiguous. You can install multiple templates and change the active one via the activate action in the Templates area, which switches the storefront to the selected template. So changing your store's look is simply activating a different installed template; staging or seasonal swaps are easy, but customers always see exactly one.