Lennart Jörgens shares his experience of building a Gatsby themes library website using Theme UI, illustrating how its features, such as breakpoints, variants, and the Tailwind preset, significantly streamlined the design and development process. He highlights the importance of starting with a design prototype in Figma, drawing inspiration from platforms like Dribbble and Behance, and maintaining consistency by using shared design tokens across both Figma and Theme UI. Jörgens emphasizes the utility of the Theme UI feature for creating responsive and customizable components, particularly through the use of variants for styling elements like buttons and gradients, and the intuitive breakpoint syntax for mobile-friendly layouts. By integrating YAML for shape positioning and leveraging Theme UI's shadowable and themeable properties, he effectively enhanced the Gatsby themes' aesthetic and functionality. Overall, Jörgens recommends Theme UI for its ergonomic and constraint-based design benefits, especially when developing Gatsby themes.