BootswatchEasy to Install Simply download a CSS file and replace the one in Bootstrap. No messing around with hex values. Customizable Changes are contained in just two SASS files, enabling further customization and ensuring forward compatibility. Tuned for 5.3.3 Themes are built for the latest version of Bootstrap. Version 4, version 3, version 2, and other releases are also available to download.
twitter-bootstrap-railsでbootswatchを使う - Qiita
