The extension code is minified but AFAICT it's just injecting some CSS sheets, primarily element hiding rules with display:none but also some spacing fixes. E.g. the Facebook one is
It's not clear why the developer chose to inject the stylesheets dynamically and magnify the footprint 10x by including a polyfill library, rather than using the manifest to specify them declaratively, but whatever.
To satisfy your curiosity: The reason I don't inject the stylesheets declaratively is because you can't disable them and I wanted to give people a way to toggle the hiding functionality.
