Black border around trac menu items

There is a black border around the menu items at the top (View Tickets, Browse Source, Roadmap, etc.) which looks like it shouldn't be there. FWICT this started after #20213 (moved) got fixed.

The CSS responsible for this border lives in https://trac.torproject.org/tor.css and is defined as follows;

#mainnav {
 background: #f7f7f7 0 0;
 border: 1px solid #000;
 font: normal 10px verdana,'Bitstream Vera Sans',helvetica,arial,sans-serif;
 margin: .66em 0 .33em;
 padding: .2em 0;
}