Simply download a CSS file and replace the one in Bootstrap. No messing around with hex values.
Changes are contained in just two SASS files, enabling further customization and ensuring forward compatibility.
Themes are built for the latest version of Bootstrap. Version 4, version 3, version 2, and other releases are also available to download.
Bootstrap themes are released under the MIT License and maintained by the community on GitHub.
Since MP3s aren't apps, "install" means transferring them to your music library.
Disclaimer: This article is for educational purposes. Always respect copyright laws. The author does not condone piracy.
Assuming you have found a legal source, here is the exact process for on Windows, Mac, and Android.
Bootswatch