Hi!
First of all, please forgive me if the issue that I’m about to describe doesn’t belong here. This seemed to be the closest applicable category.
I installed cinnamon DE. Everything seems to work EXCEPT even though I have window backgrounds and wallpapers installed and available, the system cannot use them. This same box has Plasma 5 and GNOME installed and both have no issues.
In Cinnamon icons, applications, resolution work perfectly…only on a light gray desktop. I removed Cinnamon and then replaced it hoping that something would be replaced that might have gotten “buggared”, but the same thing continued.
When Cinnamon opens and closes, I can see a GNOME wallpaper for a very brief split second.
I installed Cinnamon both as an original Desktop and on top of an XFCE Desktop (which provides a lightweight Gnome-based base system),
Trying to install Cinnamon as the original Desktop results in a broken Desktop, system boots to a text login prompt.
Installing on top of XFCE, I observe what you’ve described. I can modify Window decorations which among things changes the appearance of the application launch menu, but the background is unconfiguratble. A silver-grey solid color is applied over the underlying system background (in my case XFCE) which appears for a second or two when the Desktop loads and unloads.
Maybe eventually whoever maintains Cinnamon may put in the work to also create a proper pattern for the Desktop, today a pattern spec does not exist, it seems to be entirely defined by the main package.