I fear this will lead to a lot of broken websites. That seems to be the case with a lot of the browser plugins that provide the same functionality.
There's a mode like this in an Android Chrome fork called Cromite that works surprisingly well. Most of the other options do suck though.
1. It's opt-in. The user is choosing the occasional broken site over light themes.
2. The site can always provide their own dark theme if the automatic one is broken.