mkdir my-site
cd my-site
yarn init
# install gatsby-serenity-theme and it's dependencies
yarn add gatsby react react-dom gatsby-serenity-theme

Then add the theme to your gatsby-config.js. We'll use the long-form here for educational purposes.

module.exports = {
  plugins: [
    {
      resolve: "gatsby-serenity-theme",
      options: {},
    },
  ],
}

That's it, you can now run your gatsby site using

yarn gatsby develop

Note that this site doesn't do anything, so you're seeing a missing resources error. Create a simple page in src/pages/articles.js to see a page on the root url.

import React from "react"

export default function Home() {
  return <div>My Site!</div>
}

You can use this as a place to start when developing themes. I generally suggest using yarn workspaces like the gatsby-theme-examples repo does, but using yarn link or npm link is a viable alternative if you're not familiar with workspaces.