According to my system, there is only one file on the whole disk called userstyle.css :
~# locate userstyle.css
/etc/dokuwiki/userstyle.css
So this is the file that should cause the desired effect when I put in the statement ".dokuwiki div.footnotes {...}", but unfortunately, it doesn't. The file has the right permissions and ownership, so this can't cause the trouble.
I also tried to disable the plugin and switch back to the original footnote settings and rendering, but that doesn't cause any change either.
Do you know any statement that I can put in there that should work in any case, so I can see if the file is loaded and its contents is regarded in the first place?