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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.

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.

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.