printers: Fill the supply_frame background when supply is empty

According to the mockups at
https://wiki.gnome.org/Design/SystemSettings/Printers#Guidelines
The supply level bar should have a grey background when there are
no colors in the supply bar.

This patch sets the "background" css name to the supply_frame when
there's not a single color in the supply bar.

https://bugzilla.gnome.org/show_bug.cgi?id=779075