Bootstrap (https://getbootstrap.com) is a responsive UI framework for modern web development. It offers a mobile-first approach for web development with utilities and UI components that are fully responsive. Bootstrap 4 is the latest version, uses flexbox for layout, and is completely written in SASS, which makes it easy to customize. Bootstrap supports a 12-column grid framework, which lets you build responsive web pages with ease. JHipster uses ng-bootstrap (https://ng-bootstrap.github.io) for Angular and reactstrap (https://reactstrap.github.io/) for React so that pure Angular/React components are used instead of the ones provided by Bootstrap, which are built using jQuery, and Bootstrap is used only for styling.

JHipster also supports Bootswatch (https://bootswatch.com/) themes for Bootstrap. You can choose a theme from the supported Bootswatch themes while generating the application.