Sorry for the delay.
If a reader has an AVSForum account, it would be easy to go to the link I provided and get the CSS to install it. Then it can be tested right there on the page, or any other AVSForum page. If you don't have an account, posting it here would be a waste as the CSS is useless anywhere else.
As to your idea about a who new user profile gave me the idea to just start another instance of Vivaldi pointed at a whole new configuration location. I run two instances of Vivaldi at the moment. One that uses the default vivaldi-snapshot area and one that uses vivaldi-snapshot-2. I just started another that points to -3. Of course I had to reinstall stylebot and set it up. Once I did that, I went to some of my AVSForum pages and was well. No issues.
Now I am really confused as the only difference from when it was working to now that it isn't is that the power went out a few hours before I got home (a nasty storm went thru the area). I had shut down the other server before I left. Frankly I do not know if it was working correctly or not before the trip as I have been having password storage issues with that Vivaldi instance.
Now I get to try and figure out what happened. I am going to delete all of the cache files in the config area and see if that makes a difference. Otherwise it is going to be tough figuring this out as I do not know how the extension is handled by Vivaldi.