Hello,
The issues with thumbnails generation should be fixed in the upcoming version 14.0.
Regarding dark-mode - you mean you use some other addon for making pages look dark and you want to use it for thumbnails as well, right? The Refresh by visiting page should work fine (right?), also there will be a new refresh method that should work as well (I will add some switch to the options page to change default thumbnails generation method).

Feel free to contact me to my support e-mail to discuss this further.

Also I'm really busy with a new release so I'm replying very fast these days...

Best regards,
Juraj Mäsiar