By default, settings in rustdoc are stored as Site Data. If you're building documentation locally with cargo doc, this means that the settings will be separate for every single crate, because each crate is in its own folder and most browsers treat every folder as a different site.
This add-on fixes that, allowing multiple rustdoc pages to use the same theme, and other settings.
Available for Firefox, Chrome, and Edge.
Source code and issue tracker here.