Looks great and works pretty well. Behaviour can look a little odd on sites that are already dark but by and large I prefer my web like this.
My biggest gripe is that it introduces some performance problems on some sites. In particular I use a self-hosted site for asset management (Snipe-IT), and when searching of different assets or other items this add-on introduces lag when I type different search terms and the content of the page (the search results) dynamically changes in response.