Test_Demo_Boldin_41P/DemoTest_Boldin_41P/App.xaml

<Application x:Class="DemoTest_Boldin_41P.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:DemoTest_Boldin_41P"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <Style TargetType="TextBlock">
            <Setter Property="FontFamily" Value="Monotype Corsiva"></Setter>
        </Style>
    </Application.Resources>
</Application>