Darken the gnome icon colors to better fit our theme

Just did a find and replace on #bababa (the gnome icon color) to #2a2a2a
(a common dark color in our theme)

Also increased the opacity of some of the transparent icons somewhat
[endlessm/eos-shell#4324]
Merge pull request #13 from endlessm/shell/4324

Darken the gnome icon colors to better fit our theme