Code that changes code of another page

Hello. I use themeresources.xaml as a resource of my theme in my application. I have several themeresourcesX.xaml (x=1,2,3,4etc) Each contains other styles etc.
 
How should I program button, to change code from App.xaml.vb

       Dim source As String = [String].Format("/Exercises;component/CustomTheme/ThemeResources.xaml")

 
to

       Dim source As String = [String].Format("/Exercises;component/CustomTheme/ThemeResources2.xaml")

 
Button will be placed in Settings.xaml page.
Is that possible to change code of another page ?
 
It is a windows phone application.