You Userstyle about icons is very fine too!
:-)
Tested by copy your css and With Stylus install:
Work perfectly.

But the install as userscript (with greasemonkey 3.17) don't change these icons....
I don't understand why...
Maybe the gresyfork's conversion don't like ".onion" domains ?
I know Jason Barnabe work on it....

Test link:
Ces drôles de poissons | Pas si bêtes que ça ! | ARTE

And i tested mine as userscript:
Not working!
Hmmmm

I posted a request to Jason here:
Other problems for these userscripts...(not working with install as userscript but work with Stylus)...

As I tested, everything works fine (Greasemonkey 4.11), maybe that's your side issue? Because both works fine for me.

Maybe that's the version of greasemonkey?
Mine is 3.17, your is the last one....

Probably that's the issue. Just update then.

Yes :-)
But i don't like how work the updated version of Greasemonkey....

I tested with Tampermonkey:
All install work fine.

So i wait if my request to Jason can solve the Greasemonkey 3.XX problem.
If not, i change the description of the userscript to notice it.