Stencil framework, what can and can’t I customize?

Stencil is BigCommerce's theme framework for its native storefront. It is not the only frontend option: BigCommerce also supports headless approaches such as Catalyst. A custom frontend does not grant direct control over the hosted commerce engine. Locate your proposed change first: presentation, supported checkout extension, external app or backend business rule. Demonstrate it with the actual APIs and plan, including fallback behavior, before treating a framework choice as proof of fit.

Official references: BigCommerce storefront choices.