Eventually cracked it.
There are 4 folders of the form YYYY.default\
3 of them had a sub-folder chrome, each of which had 2 files - userChrome-example.css and userContent-example.css. Creating a userChrome.css file in any of these 3 made no difference.
The fourth folder had no chrome sub-folder so I created one, created userChrome.css and added the code from the aforementioned link.
Then it worked.
Relevant or otherwise the fourth folder had a name of YYYY.default-aaaa (where aaaa represents 13 digits). The other 3 are, presumably, old profiles.
We have more and more laws, and less and less enforcement