I can't seem to get it to work. Using Greasemonkey on Firefox/Windows. I've tried disabling all other scripts and styles to make sure there's not something conflicting. I don't see anything coming up like the demo picture. I've got one message currently unread to test it with.
I found the culprit, I somehow messed up and the userscript won't work if you also don't have the subscriptions panel userscript on, i will find a fix and release an update asap
The culprit in my case was Greasemonkey. It's the second issue I've had with it and a Kbin script, and this one didn't seem fixable based on some Googling*, so I went ahead and moved all the scripts to Violentmonkey instead. Works now.
*the problem was the GM_addstyle, seems to be a depreciated/broken thing with Greasemonkey, and what hacks I found didn't seem to be worth pursuing.