Change color of signature and tar icons.

Change color of signature and tar icons.

These icons are currently black, which looks nice on the white
background of the releases page. However, all other pages use a black
background, and it would be nice to extend this to the releases page
page as well (by teaching it to honor our para.css file). As a first
step, switch to grey icon colors as these look nice on both a black
and a white background.