Rated 5 out of 5 stars

Great add-on! Works absolutely amazing with the Thunderbird dark theme and prevents burning my retinas when opening emails. Your add-on seems like a simplified version of Dark Reader, isn't it?

Yes, something like that. The "Dark Reader" is much more sophisticated (but also CPU hungry). The color inversion is actually quite complex thing.
I'm using only basic techniques here. When I have more time, I plan to upgrade it so that it works better with "already dark" and "partially dark" emails. But it won't be easy... this is not exactly my area of expertise.

Still, I'm surprised how well it works considering how fast I've build it :). It makes me happy every time I open email in the evening :)

Best regards,
Juraj Mäsiar