gatsby new my-theme https://github.com/gatsbyjs/gatsby-starter-theme-workspace
cd my-theme
yarn workspace example develop

Layout

.
├── README.md
├── gatsby-theme-minimal
│   ├── README.md
│   ├── gatsby-config.js
│   ├── index.js
│   └── package.json
├── example
│   ├── README.md
│   ├── gatsby-config.js
│   ├── package.json
│   └── src
├── package.json
└── yarn.lock

3 directories, 10 files

gatsby-theme-minimal

This directory is the theme package itself. You should rename this at some point to be gatsby-theme-{my-theme-name}. Also change the package.json name field and the corresponding dependency in the example directory's package.json/gatsby-config.js to match the chosen name.

• gatsby-theme-minimal/
  • gatsby-config.js: An empty gatsby-config that you can use as a starting point for building functionality into your theme.
  • index.js: Since themes also function as plugins, this is an empty file that gatsby needs to use this theme as a plugin.
  • package.json: The dependencies that your theme will pull in when people install it. gatsby should be a peerDependency.

example

This is an example usage of your theme. It should look the same as the site of someone who installed and used your theme from npm.

• example/
  • gatsby-config.js: Specifies which theme to use and any other one-off config a site might need.
  • src/: Source code such as one-off pages or components that might live in a user's site.

You can run the example with:

yarn workspace example develop