The bigger question is; why would you want to subject everyone to seeing a nasty sherbert/sherbet green background on all your posts?This would not affect anyone that views the web page but me. That's how Stylus works.
I think he was being funny. :)
Gotcha - I sped read your OP and mostly read the last sentence so I didn’t catch the Stylus FF addons part.
Don’t think it’s doable. The divs are a2, b2 etc but there’s no div specific to an individual FR user’s reply. I see what you’re wanting though. Something similar to your My Comments page but with green background instead of green text. That’s done with a <li - list item with a class=yourself but only applies to the My Comments page. I don’t see anything user based to grab onto like that for regular thread replies though.