Hello,
as the dev is lazy and not replying to subjects of their users I made a 5 min darkmode which is dark and ugly, when i am more into it there will be a decent version of the whole extension on Github.
So here you go:
1. Locate the extension folder - respectively what browser you using the extension, for me it is Opera, get your profile path and navigate to the extension folder ID - edacconmaakjimmfgnblocblbcdcpbko
Example: C:\Users\User\AppData\Roaming\Opera Software\Opera Stable\Extensions\edacconmaakjimmfgnblocblbcdcpbko
2. Go into folder (version may be difference) and navigate to css folder
3. replace the main.css and sbButton.css with the attached ones
4. IMPORTANT - make rightclick on files and select copy protected, so that the files does not get replaced by some browser mechanism, which checksum the files and thinks it could be an infection.
5. now you have a ugly darkmode
have fun!