Gatsby has announced the stable release of Gatsby Themes, which allows developers to abstract their site's default configurations into an installable package, making it easy to update, compose, and swap themes. This release moves the theme APIs from an experimental phase to stable status, with composition and shadowing as core features. Composition combines configurations from various plugins and the site's gatsby-config, whereas shadowing allows developers to override specific components by creating new files. The previously experimental __experimentalThemes key is deprecated, and themes are now integrated into Gatsby's core functionality. Gatsby has also launched two official themes, gatsby-theme-blog and gatsby-theme-notes, built with the Theme UI library, which encapsulates best practices for theme development. Future plans include developing tools to make features like shadowing more accessible through the Gatsby CLI and GUI, enhancing the ease of working with themes.