User12/WpfApp1/App.xaml

<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <Style TargetType="TextBlock">
            <Setter Property="FontFamily" Value="Monotype Corsiva" />
            <Setter Property="FontSize" Value="20"/>
        </Style>

        
        
    </Application.Resources>
</Application>