Background color turns black when changing display settings

Steps to reproduce:

- Start Tails 1.1.

- Change display resolution in Applications → System Tools → Preferences
→ System Settings → Displays.
- Result: the background color turns black.

This happens has well if I set the background color using the GUI
utility for that. But this doesn’t happen if I set an image as wallpaper.

I can reproduce that both on bare metal and in VirtualBox.

Feature Branch: bugfix/7248-bg-color-vs-display-settings-change

Original created by @sajolida on 7248 (Redmine)

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