[Plugin: NextGen gallery] customize shutter nav buttons

I’m using NextGen gallery; I have several image list galleries and I’d like to change the appearance of “prev” and “next” buttons when the full size images are displayed (after clicking on a thmbnail). I’m using the shutter.
I found how to customize everything (in the shutter/shutter_reloaded.css) but I’m definitely not able to find how to change the ? (next) and ? (prev) symbols.
Dan anyone help?
Thanks,
andrea