[Bug 79614] Add soft background color behind Control Center's headers

[Eugenia Loli-Queru]

Public bug reported:

Binary package hint: gnome-control-center

Please consider adding a soft grey color behind the headers, because it
makes it much easier to the eyes and brain to quickly identify what is
what and where. Mockup here:
http://bugzilla.gnome.org/attachment.cgi?id=71122&action=view

** Affects: control-center (Ubuntu)
     Importance: Undecided
         Status: Unconfirmed

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Add soft background color behind Control Center's headers
https://launchpad.net/bugs/79614