Oops I just noticed I had a typo in the path, but it looks like yours is correct.
I installed joplin on a windows computer to test this and the userstyle.css file worked correctly with the path you specified, but not with the css you provided. I’m not an expert on css but I think the
a:visited doesn’t work with userstyle.css, perhaps because userstyle stuff is basically inline, or it could have to do with the renderer joplin uses.
That said, you can still customize links, but it looks like you have an issue with your css, there should not be a colon after the ‘
This worked on my computer, but what you have above didn’t.