The current export button uses an image with a hardcoded image path:
alt="Download CSV File"
title="Download CSV File" height="14" width="36">
That is hard for a themer to overwrite; you'd have to use jQuery to manipulate the dom directly.
It would be great if this was stylable instead with CSS. Instead of the imgage tag, you could use some inner divs and then set the background image using CSS.