Kirigami.Window colorset for pages full of cards

Cards make sense when they have a bit of contrast against the background they are on top.
The homepage and the application list have white cards on white background which look a
bit glaring, on those pages, and only those, there is a definite improvement if the background
in from the Window color set

before:

![image](/uploads/6a0de31e606b9913733e8f43f5773dfd/image.png)
![image](/uploads/d958a3bb000e40647aa34aee210d3560/image.png)

after:

![image](/uploads/39d6a50e4a38655dd67a14c6a9fe2678/image.png)
![image](/uploads/656e84101a127935b8b26e823ef9ca79/image.png)