Gatsby themes offer a way to share functionality across multiple Gatsby sites, and using Yarn workspaces for theme development provides an efficient setup by organizing multiple packages within a single directory and linking dependencies. This method allows developers to maintain both themes and example sites together, streamlining the development process. The setup begins with installing Yarn and creating a directory structure with separate subdirectories for the theme and an example site, each initialized with its own `package.json`. It is crucial to ensure that the theme's `package.json` name matches its directory name due to Gatsby theme shadowing requirements. By utilizing Yarn commands, developers can install necessary dependencies and manage them across workspaces, while also setting up a basic theme environment with configuration files and demo pages. This approach facilitates the testing of themes in a local development setting, ensuring compatibility and functionality before publishing. The guide emphasizes exploring further resources and posts for in-depth Gatsby theme development.