<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <x:Double x:Key="SecondaryTextFontSize">12</x:Double>
    <Style x:Key="SecondaryTextStyle" TargetType="TextBlock">
        <Setter Property="FontSize" Value="{StaticResource SecondaryTextFontSize}" />
        <Setter Property="Foreground" Value="{ThemeResource TextFillColorSecondaryBrush}" />
    </Style>
</ResourceDictionary>