Create a theme in Windows 8.1 - xaml

Question

In Windows 8, you were able to create your own themes for your application (here's a tutorial).
In Windows 8.1 Applications, themes are handled differently: you can change them at run-time and set a theme for a specific control in your XAML (if you don't want to apply the theme to the whole Application).
For instance:
<Grid x:Name="MainGrid" RequestedTheme="Dark">
However, I could not find a way to create my own themes. The property RequestedTheme takes an enumeration (its type is FrameworkElement.RequestedTheme) and an enumeration by definition cannot be extended (in C#). Also, if I want to define a new Theme Dictionary I would have written:
<ResourceDictionary.ThemeDictionaries>
But it is not available in Windows 8.1.
How can one create a theme in Windows 8.1? Am I limited to the existing ones (light & dark)?

Answer 1

Yes you're restricted to 3 themes I believe
Default (light)
Dark
High Contrast
You can create new styles or override existing ones for the 3 themes like this in 8.1
<ResourceDictionary.ThemeDictionaries>
<ResourceDictionary x:Key="Default">
<Style TargetType="TextBlock">
<Setter Property="FontSize" Value="24" />
<Setter Property="Foreground" Value="Green"/>
</Style>
</ResourceDictionary>
<ResourceDictionary x:Key="Dark">
<Style TargetType="TextBlock">
<Setter Property="FontSize" Value="30" />
<Setter Property="Foreground" Value="Orange"/>
</Style>
</ResourceDictionary>
<ResourceDictionary x:Key="HighContrast">
<Style TargetType="TextBlock">
<Setter Property="FontSize" Value="24" />
<Setter Property="Foreground" Value="Blue"/>
</Style>
</ResourceDictionary>
</ResourceDictionary.ThemeDictionaries>