An inspector tools panel. Tinker with a site's CSS in real time, automatically remember your changes.
TinkerCSS appends a CSS style tag to the end of a HTML document <head>, that you can then live modify via the extension's editor Inspector panel, and see the results in real-time.
The CSS you write is saved with reference to the page URL, so if you open the same page again and load the panel, your previously written snippet will load up.