Bootswatch is a gallery of free themes for the Bootstrap CSS framework. Each theme is a drop-in CSS file that restyles a Bootstrap site without touching its markup.

Theme collection - Ready-made styles such as Brite, Cerulean, Cyborg, Darkly, Flatly, Minty, Sketchy, and Vapor, each with a live preview.
Easy install - Download a CSS file and replace the default Bootstrap stylesheet.
Customizable - Changes live in two SASS files, so themes can be adjusted and kept compatible with future releases.
Multiple Bootstrap versions - Themes built for Bootstrap 5.3, with older lines for versions 4, 3, and 2.
Open source - Released under the MIT License and maintained by the community on GitHub.
Integration API - An API for pulling themes into other platforms, used by tools such as NodeBB.

It is used by web developers and designers who build with Bootstrap and want a different look without writing custom CSS from scratch.

All themes are free and open source.