Material UI

Material UI is a React UI library based on Google's Material Design. As they write: "Material Design is inspired by the physical world and its textures, including how they reflect light and cast shadows. Material surfaces reimagine the mediums of paper and ink."

The library itself provides a collection of components that are styled based on this design. Its advantage is that it's really easy to create a frontend with modern design even without CSS knowledge. The components can be customized with JavaScript because it uses Emotion JS. However, it's rarely necessary because the most common customizations are available through component props.This architecture works so well that the entire project has 15 lines of CSS code which is used to customize the scrollbar.

Also since it's very popular there's also a library for react native which helps to keep the web design and the application design uniformly.