Theming and styling JSS apps
Prior to the introduction of rendering SDKs, theming and styling of Sitecore front-end applications resided in Sitecore.
While Content Authors retain control over page and component layouts, front-end developers building applications with JSS are responsible for the look and feel of the JavaScript applications.
JSS is flexible about which styling methods or tools to use. To set up styling tools for your JSS application follow the best practices and use the CSS tools and frameworks that suit your requirements. Refer to official framework documentation and community tutorials for how to set up your CSS tooling for the framework you chose to implement JSS applications.
Next.js has built-in CSS support and the documentation provides examples of how to set up various styling tools for a Next.js application.