UP_05_04/Esoft/App.xaml

<Application x:Class="Esoft.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:Esoft"
             StartupUri="Pages/Authorization.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Dictionary.xaml"/>
            </ResourceDictionary.MergedDictionaries>

            <!-- Добавляем глобальный стиль для TextBlock -->
            <Style TargetType="TextBlock">
                <Setter Property="FontFamily" Value="Arial Black"/>
                <Setter Property="FontSize" Value="20"/>
            </Style>

            <Style TargetType="Button">
                <Setter Property="FontFamily" Value="Arial Black"/>
                <Setter Property="FontSize" Value="20"/>
            </Style>

        </ResourceDictionary>
    </Application.Resources>
</Application>