Excellent tool for darking websites but unfortunately can be a bit slow at applying (down to how it works) and so blinds you with a flash of white before it loads. Same does apply to the new tab page which would be nice if Mozilla could find a way to darken that properly.

That said when it does work, it has a great effect on darkening a site in a good way. It would be lovely if there was the option of exporting a stylesheet of a darkened site using this.