Question: how to change background color in xaml?
Answer: Create a new custom theme based on the one you want to modify.
Click the "Show All Elements" filter button in the upper-left of the theme editor.
Type "artboard" in the search-box in the upper-right of the theme editor.
Set the " Cider -> ArtboardBackground " color to a different color of your choice.
Yay! : D.