I think I'm out of ideas then I'm afraid, it sounds like Joplin is loading the CSS just fine and I'm not aware of other reports of this nature (I was playing with userstyle on a windows PC just the other day and it was fine).

The only other thing I can suggest is to post the log files here and hope there is something else or if anyone else has any bright ideas as to what is wrong.

And just to check as this has come up before, you definitely are using the markdown editor/viewer mode and not the richtext editor aren't you?