Idea: refactor ColorPageExample to use SolidBackgroundFillColorQuarternaryBrush by default

[Jay-o-Way]

Every time ColorPageExample is used, the Background is defined, most often with SolidBackgroundFillColorQuarternaryBrush. I want to use this value as a default, so there is less need to define it for every single use. But the it also needs to accept a value - just like it does now - to be able to override it.