Use our usual background color in the Greeter

Originally created by @intrigeri on #7126 (Redmine)

On the Wheezy builds, the Greeter displays the default Debian Wheezy’s background image. We should replace it with the usual blue color that is used on the desktop.

Feature Branch: bugfix/7126-gdm-background

Related issues

Blocks #7187 (closed)

Edited May 15, 2020 by intrigeri

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