The Automations tab of the Listing Designer (Design, then Listing Designer, then Automations) applies a default design to eBay listings you create from now on: turn on Automatically apply a design to newly created listings, choose the design, and optionally click Add Rule to set a design for a category. Every change on this tab saves immediately, and existing listings are not changed.
Full guide with screenshots: Apply, Reapply and Remove Listing Designs on eBay (section "How do I apply a design to every new listing automatically?").