Why UWP ApplicationPageBackgroundThemeBrush is always white?

I eventually find out the problem by googling about. The problem is caused by the VS2015 project template. In app.xaml, there is a line to set RequestedTheme="Light". I removed the line and things are fine now. Wasted me 2 hours. Hope you see my answer and therefore save time.

https://social.msdn.microsoft.com/Forums/vstudio/en-US/c12cdba4-093f-474a-9d21-6e447aaa2adf/uwp-applicationpagebackgroundthemebrush-is-always-white?forum=wpdevelop