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Does not work in thunderbird 115.3.3. No effect on message pane.
Update Oct 20: Followed the developer's instructions, and now the extension works. Might be a miracle ... but hey,

Это отзыв для предыдущей версии этого дополнения (3.6). 

Hello,
Thunderbird sometimes breaks addons...
You can try to uninstall Darko, restart Thunderbird and install it back. That may help.
Alternatively, there is another (probably even better) dark mode addon - Dark Reader which may work.

Best Regards,
Juraj Mäsiar