You can define a default theme for your app, in WP 8.1.
In Windows Phone, there are two built in themes: “Light” and “Dark”. By default the system runs using the “Dark” theme. But the user can change this to use the “Light” theme.
So, whether your app has a completely custom UI, you might find some issues with components when the theme changes. Some text disappears and colours don’t contrast. To solve this, you can set the page’s RequestedTheme property to specify which theme is used.
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" RequestedTheme="Dark">
You can use this property in the App.xaml file as well. This way your app will always use the settings (backgrounds, fonts, foregrounds, etc.) of the theme you’ve defined.
This also avoids the need to override properties and create styles in XAML.
Don’t forget to read the documentation for this at MSDN.