What are your thoughts about JSS?

JSS is a CSSinJS library http://cssinjs.org/.

JSS is a better abstraction over CSS. It uses JavaScript as a language to describe styles in a declarative and maintainable way. It is a high performance JS to CSS compiler which works at runtime and server-side. You can use it with React or with any other library. It is about 5KB (minfied and gzipped) and is extensible via plugins API.