userstyle.css won’t work unless the encoding is set to UTF-8.
If the encoding of userstyle.css were ANSI or Unicode, the temporary css generated and actually applied to the markdown render is padded out with empty spaces for letter by letter, making the css file invalid.
Changing the encoding to UTF-8 got it working. Maybe this is a bug, or perhaps at least mention it in the docs; as this issue could easily trip people up? thanks.