Theming a Xamarin.Forms application
Theme an application
Theming can be implemented in Xamarin.Forms applications by creating a ResourceDictionary
for each theme, and then loading the resources with the DynamicResource
markup extension.
Respond to system theme changes
Devices typically include light and dark themes, which each refer to a broad set of appearance preferences that can be set at the operating system level. Applications should respect these system themes, and respond immediately when the system theme changes.