Is that CSS is for a horizontal rule (<hr />)? If it is, remove the padding, that makes it 1em + 1px [Oops my mistake, that should be 1px +1em +1em +1px]
high. Use this,
This will make the line 1px high.
A single CSS error will break all the formatting on all ADE based e-readers (Nook, Sony, Kobo), that is probably why the Pubit preview was not showing any formating. Use Adobe Digital Editions
to preview your epubs to catch these errors. I don't know how many times I've pulled my hair out over a dropped semi-colon in my CSS when I started creating epubs.
Added thought, the original appeared to work in Sigil and Calibre because of the missing semi-colon, it ignored everything after this, margin: 0em 0em 0em 0em in that line.