SetBackground does not work together with ToLightTheme

[martinsuchan]

In my WP8 app I use the ToLightTheme to have light theme in my app no matter what color scheme is set by the user. I also use the SetBackground method for setting light blue color as a Frame background color.
The problem is if user has while theme selected, then my custom background color is not applied, probably because the Theme Manager thinks that ToLightTheme does not need to be applied if white theme is already on.

If SetBackground is used, it should not be ignored in ToLightTheme or ToDarkTheme methods.