TicTacToe/tictactoe/App.xaml

<Application x:Class="TicTacToe.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:TicTacToe"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

        <Style TargetType="Button" x:Key="Field">
            <Setter Property="FontSize" Value="36" />
            <Setter Property="HorizontalAlignment" Value="Stretch" />
            <Setter Property="VerticalAlignment" Value="Stretch" />
        </Style>
        
    </Application.Resources>
</Application>