title: ThemeResources not resolved in Windows Phone XAML
body: <p>Here is my problem:</p>
  
  <p>In my application for WP 8.1, I have to make a button with an image, and this image have to change according to the theme used in the device,otherway it will be hidden by the background.</p>
  
  <p>So I've tryed using ThemeResource. But it can't be resolved.</p>
  
  <p>Here is the code:</p>
  
  <p>MainPage.xaml</p>
  
  <pre><code>&lt;Page.Resources&gt;
      &lt;ResourceDictionary&gt;  
          &lt;ResourceDictionary.MergedDictionaries&gt;
              &lt;ResourceDictionary Source="ButtonImage.xaml"/&gt;
          &lt;/ResourceDictionary.MergedDictionaries&gt;
  
  ....
  
  &lt;Button HorizontalAlignment="Left"
          VerticalAlignment="Top" 
          Width="40" Height="40"
          MinWidth="0" MinHeight="0"
          Padding="0"
          BorderThickness="1.5"
          Grid.Column="2"
          Style="{ThemeResource Button123}"/&gt;
  </code></pre>
  
  <p>ButtonImage.xaml</p>
  
  <pre><code>&lt;Style x:Key="ButtonImage" TargetType="Button"&gt;
  ...
  &lt;/Style&gt;
  
  &lt;ResourceDictionary.ThemeDictionaries&gt;
      &lt;ResourceDictionary x:Name="Black" x:Key="Black"&gt;
          &lt;Style x:Key="ButtonAZ" BasedOn="{StaticResource ButtonImage}" TargetType="Button"&gt;
              &lt;Setter Property="Content"&gt;
                  &lt;Setter.Value&gt;
                      &lt;Image Source="Assets/AtoZWhite.png"&gt;&lt;/Image&gt;
                  &lt;/Setter.Value&gt;
              &lt;/Setter&gt;
          &lt;/Style&gt;
          &lt;Style x:Key="Button123" BasedOn="{StaticResource ButtonImage}" TargetType="Button"&gt;
              &lt;Setter Property="Content"&gt;
                  &lt;Setter.Value&gt;
                      &lt;Image Source="Assets/123White.png"&gt;&lt;/Image&gt;
                  &lt;/Setter.Value&gt;
              &lt;/Setter&gt;
          &lt;/Style&gt;
      &lt;/ResourceDictionary&gt;
      &lt;ResourceDictionary x:Name="White" x:Key="White"&gt;
          &lt;Style x:Key="ButtonAZ" BasedOn="{StaticResource ButtonImage}" TargetType="Button"&gt;
              &lt;Setter Property="Content"&gt;
                  &lt;Setter.Value&gt;
                      &lt;Image Source="Assets/AtoZBlack.png"&gt;&lt;/Image&gt;
                  &lt;/Setter.Value&gt;
              &lt;/Setter&gt;
          &lt;/Style&gt;
          &lt;Style x:Key="Button123" BasedOn="{StaticResource ButtonImage}" TargetType="Button"&gt;
              &lt;Setter Property="Content"&gt;
                  &lt;Setter.Value&gt;
                      &lt;Image Source="Assets/123Black.png"&gt;&lt;/Image&gt;
                  &lt;/Setter.Value&gt;
              &lt;/Setter&gt;
          &lt;/Style&gt;
      &lt;/ResourceDictionary&gt;
  &lt;/ResourceDictionary.ThemeDictionaries&gt;
  </code></pre>
  
  <p></p>
  
  <p>Can some one tell me why is impossibile to use the themeResources??</p>