Follow up:
I couldn't find a way to change the color on the Gallery2.Template.php as, for my interpretation it uses the default for the boxes that are defined on the index.css.
On the other hand, I didn't managed to change the color by editing, on what seemed logic to me, the index.css.
I'm going to try to find those boxes on the code and change them to other kind as see if I manage to make it work...
