Slice/StoreServer/App.xaml

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