Engineering Better Next.js Themes: A Code Audit of Three Libraries

This comprehensive code audit reveals that all three major Next.js theme libraries ship with subtle production bugs that users notice but developers struggle to debug—issues like FOUC (Flash of Unstyled Content), React 19 compatibility problems, and XSS vulnerabilities in inline scripts. The core challenge that defines every theme library is applying the correct theme before any pixels paint, requiring an anti-FOUC inline script that runs synchronously before React hydration. Understanding this architectural constraint and the specific bugs in popular solutions is essential for building or maintaining reliable theme libraries in production.