Theme does not change instantly like a regular ResourceDictionary when inherited

[RabhiaBehamK]

Description
When inherited ResourceDictionary and create a new class the theme does not change instantly as a regular ResourceDictionary does.

Steps to Reproduce
Run the below sample in Xamarin.Forms
On the Switch,
BackgroundTheme should be Red ad LightTheme has been applied.
Navigate to the second page.
BackgroundColor is changed to Red.

Expected Behavior
BackgroundTheme should be Red ad LightTheme has been applied in the current page itself.

Actual Behavior
BackgroundTheme is applied Red ad LightTheme has been applied in the next navigated page only.

Basic Information
Xamarin: 4.7.0.968

Reproduction Link
ThemeApplication.zip