Gatsby themes provide a streamlined way to build websites and applications by packaging parts of the process into reusable components, allowing developers to focus on customized elements. The guide by Katie Fujihara explains how to set up a Gatsby theme project using the gatsby-plugin-mdx package, which facilitates embedding JSX within markdown for enhanced component creation. It details the process of organizing project directories, configuring package.json files, and setting up dependencies like Gatsby, React, and ReactDOM. The tutorial also emphasizes customizing themes by creating styled components and configuring layout options in the gatsby-config.js file. Additionally, it covers troubleshooting plugin errors and encourages sharing themes on platforms like GitHub for reuse in other projects. Fujihara invites readers to share their theme-building experiences on Twitter, reflecting her passion for creating attractive user interfaces and engaging with the tech community.