Headaches with configuring themed resources from c# (Windows Phone) - silverlight

Question

I have Windows Phone App and I want to have a different themed resources based on whether the user has a dark or light theme and which accent color they have chosen.
In my app initialize code I detect the users theme and accent color then load the appropriate Resource Dictionary, Dark, Light etc. I add the Resource Dictionary to the App.Current.Resources.MergedDictionaries collection.
The problem comes when I want to reference a value in the (dynamically loaded) from my App.Xaml. In the below example the key is "DefaultBackgroundImageOpacity". For some reason the values in the ResourceDictionary that I load into the MergedDictionaries collection never get found when the App.Xaml parsing happens. I've played with loading the resources before I call InitializeComponent() and after. Neither seem to resolve the issue.
Any ideas?
// Simplified version of adding some xaml...
// Note I'm loading the key "DefaultBackgroundImageOpacity"
this.Resources.MergedDictionaries.Clear();
var myTestXaml = "<ResourceDictionary xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' xmlns:System='clr-namespace:System;assembly=mscorlib'> <System:Double x:Key='DefaultBackgroundImageOpacity'>0.2</System:Double></ResourceDictionary>";
this.Resources.MergedDictionaries.Add((ResourceDictionary)XamlReader.Load(myTestXaml));
// This always fails saying that the key "DefaultBackgroundImageOpacity"
// can not be foudnd - even though it was loaded just above
InitializeComponent();
// Phone-specific initialization
InitializePhoneApplication();

Answer 1

I suspect your App.Xaml contains something like this:-
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/somedictionary.xaml" />
...
</ResourceDictionary.MergedDictionaries>
...
</ResourceDictionary>
</Application.Resources>
If that is so then InitializeComponent will replace the default ResourceDictionary which you have manipulated in your code hence your changes to MergeDictionaries will not be present since that instance of a ResourceDictionary is no longer referenced.
If you are not setting up any MergedDictionaries in the App.Xaml then make sure you add resources directly to Applicaiton.Resources and are not creating an new instance of ResourceDictionary in the xaml.