This add-on does apply a dark theme to Amazon, but it also quietly rewrites product links to insert the developer’s Amazon affiliate tag. This behavior is not clearly disclosed and has nothing to do with dark mode.

It intercepts clicks and adds the tag automatically, sometimes only once every 15 minutes, which makes it harder to notice. There is no obvious way to disable this.

Using affiliate links is fine when it’s transparent. Doing it silently is not. If you just want dark mode, use a different extension.