Gatsby themes aim to provide a flexible and extensible framework for building sites by allowing the reuse of site configurations, plugins, and components across multiple sites, with a focus on progressive complexity and easy customization. The release of two official themes—a blog theme and a notes theme—demonstrates this flexibility by allowing users to customize typography, color, layout, and other design tokens through a theme configuration object. Theme UI, an optional open-source library developed alongside these themes, enhances customization by enabling consistent styles across React components and simplifying the styling of MDX content. It builds upon the Styled System library to offer portable configuration and guidelines for creating themeable UI without starting from scratch. Users can implement custom styles by leveraging Gatsby's shadowing feature and the Theme UI library, with the potential to integrate additional tools and libraries for enhanced site styling.