title: Xamarin.Forms: How to set values in Style only on specific platform
body: <p>In a style in xamarin forms (defined in xaml) I can define values platform specific, like this:</p>
  
  <pre><code>&lt;OnPlatform x:TypeArguments="Font" Android="30" iOS="40" WinPhone="20" x:Key="TitleFontSize" /&gt;
  
  &lt;Style x:Key="MyTitleLabel" TargetType="Label""&gt;
      &lt;Setter Property="Font" Value="{StaticResource TitleFontSize}" /&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>Can I also define the style in a way, so that the value of "Font" is kept to the default (not set at all) for some plarforms?</p>
  
  <p>Something like this:</p>
  
  <pre><code>&lt;Style x:Key="MyTitleLabel" TargetType="Label""&gt;
      &lt;OnPlatform&gt;
      &lt;OnPlatform.Android&gt;
          &lt;Setter Property="Font" Value="30" /&gt;
      &lt;/OnPlatform.Androud&gt;
      &lt;/OnPlatform&gt;
  &lt;/Style&gt;
  </code></pre>