bootstrapper is a Grav plugin that can be used as a dependency for other themes and plugins to load Bootstrap Framework assets. The main purpose of this plugins is to allow the Boostrap theme to depend on the bootstrap CSS/JS and allow the plugin to be updated independently of the theme itself. If for some reason you can't use GPM you can manually install this plugin. Download the zip version of this repository and unzip it under /your/site/grav/user/plugins. Then, rename the folder to bootstrapper.