Rated 5 out of 5 stars

Thank you.
----------------
Dear developer,

This add-on is great.
I want you to support Thunderbird Ver100.0b4 or later.
Thank you.

This review is for a previous version of the add-on (3.3). 

Hello,
It should work fine in the current Beta and also Alpha 101.
But Thunderbird is a bit broken sometimes, you can try to uninstall it, restart Thunderbird and install it back.
Alternatively, there is another (probably even better) dark mode addon - Dark Reader which may work.

Best Regards,
Juraj Mäsiar