colors: fixed background

Fixed background color in Contacts.List
Fixed background color in "Select a contact" view

Fixes bug: https://bugzilla.gnome.org/show_bug.cgi?id=705972
ContactPane: refine select of Select a contact

Added css class to match the mockup
Added some margin_bottom to the box as hack to try and center
vertically the watermark

Note: This one requires of a patch against gnome-themes-standard