Gatsby is a powerful static site generator built with React.js, Webpack, and GraphQL, designed for creating fast websites with static content such as blogs, eCommerce sites, and company websites. This guide explores the process of building a Gatsby theme, which differs from a starter by serving as a library that can be updated and used to streamline the setup of a user interface. The process involves creating a theme directory, installing necessary dependencies, and exporting components for use in a site while allowing for component shadowing to customize theme features. The use of MDX, which combines markdown syntax with JSX for more advanced components, is highlighted as a key feature, along with methods for importing components and styling. The guide also covers the concept of using design tokens for default theme styling that can be overridden, and the application of component shadowing to modify theme parts. Additionally, the text discusses the possibility of using multiple themes simultaneously and addresses common errors encountered during theme creation, with a nod to future advancements in Gatsby themes.