Suggestion from "dcent" at gnusocial.net (I've applied it and it works fine):
In 'neo-quitter' theme, which have the elegant "*gnu*social" logo in the top-left, CSS needs to be:
background-position: -10px -601px; /* not -600px */
Ie. 1 pixel upward so the transparent(?) part in the image doesn't show a thin line artifact at the top of the element.
@spookie, can you have a look at this?