MessageBoxDialog.xaml

<Window x:Class="DotNetPad.MessageBoxDialog"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DotNetPad"
        mc:Ignorable="d" WindowStartupLocation="CenterOwner"        
        ShowInTaskbar="False" ResizeMode="CanResize" WindowStyle="None" Loaded="Window_Loaded"
        SizeToContent="WidthAndHeight" BorderThickness="0" Background="Transparent" MinWidth="300" MaxWidth="400">
    <Grid HorizontalAlignment="Stretch" VerticalAlignment="Center">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid Grid.Row="0" Name="TopGrid" HorizontalAlignment="Stretch" VerticalAlignment="Center">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="25" />
                <ColumnDefinition Width="*" MinWidth="300" MaxWidth="400" />
            <ColumnDefinition Width="25" />
            </Grid.ColumnDefinitions>
            <Grid Grid.Column="1" HorizontalAlignment="Stretch" VerticalAlignment="Center">
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto" />
                    <RowDefinition Height="Auto" />
                </Grid.RowDefinitions>
                <TextBlock Name="TitleTextBlock" Grid.Row="0" Height="Auto" Text=".NETpad" 
                           Style="{DynamicResource SubtitleTextBlockStyle}" Margin="0,25,0,10" />
                <TextBlock Name="MessageTextBlock" Grid.Row="1" Text="Message text" 
                           Style="{DynamicResource BodyTextBlockStyle}" Margin="0,0,10,0" />
            </Grid>
        </Grid>
        <Border Grid.Row="1" Name="BottomBorder" BorderThickness="0,1,0,0" HorizontalAlignment="Stretch" 
                VerticalAlignment="Top" Margin="0,25,0,0" BorderBrush="{DynamicResource DividerStrokeColorDefaultBrush}" Background="{DynamicResource LayerOnMicaBaseAltFillColorSecondaryBrush}" >
            <Grid Height="Auto" VerticalAlignment="Center" HorizontalAlignment="Stretch">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="25" />
                    <ColumnDefinition Width="*" MinWidth="300" MaxWidth="400" />
                    <ColumnDefinition Width="25" />
                </Grid.ColumnDefinitions>
                <Grid Grid.Column="1" Name="ButtonGrid" HorizontalAlignment="Stretch" VerticalAlignment="Center">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition MinWidth="300" MaxWidth="400" Width="1*" />
                    </Grid.ColumnDefinitions>
                    <Button Grid.Column="0" Content="OK" Width="200" Height="32" MinHeight="32" Focusable="True" 
                                HorizontalAlignment="Right" Margin="0,20,10,20" Name="MessageBoxOKButton" 
                                Click="MessageBoxButton_Click" TabIndex="0" IsDefault="True" IsCancel="True"
                                Style="{DynamicResource AccentButtonStyle}" />
                </Grid>
            </Grid>
        </Border>
    </Grid>
</Window>