Rated 5 out of 5 stars

Amazing, does a perfect job of changing the email appearance while still allowing me to use any theme I want. One suggestion I have is matching the dark background color with the dark grey used in the thunderbird dark theme.

Edit: Ah, I see. I think the text email backgrounds match the color very well but emails with a built in white background get totally inverted to black I think if that makes sense.

This review is for a previous version of the add-on (2.0). 

Hello,
I can add new option to change background color. Currently it matches the background color used by Thunderbird in the Compose window (when using default dark theme).

Note that custom background color will work only on emails without background color because that is being inverted by the filter and I can't change it (without breaking the rest of the colors).

I will look at it during the weekend, it should be then part of the 2.1 version.
Here the issue for tracking:
https://github.com/fastaddons/Darko/issues/4

Best regards,
Juraj Mäsiar