boxes-empty-image: Use inherited color

The hardcoded color doesnt play well with the new background. This makes
the image better visible while having less hardcoded colors as a side
effect.

https://bugzilla.gnome.org/show_bug.cgi?id=735153
gtk-style: Remove .boxes-empty-details-label

The .boxes-empty-details-label class is redundant since it only sets
color to improve readability while it ends up doing the opposite since
the restyle through commits 88d1119c to 787852bc. It is therefore of
no use to have this class anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=735153