using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Pr
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Ok_Click(object sender, RoutedEventArgs e)
        {

        }

        private void Close_Click(object sender, RoutedEventArgs e)
        {

        }
    }
}

<Window x:Class="Pr.MainWindow"
        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:Pr"
        mc:Ignorable="d"
        Title="MainWindow" Height="448" Width="257" VerticalAlignment="Bottom" Margin="0,0,0,0" AllowsTransparency="True" Background="Transparent" WindowStyle="None" ResizeMode="NoResize" Topmost="True">
    <Grid>

        <Border Width="Auto" Height="Auto" Name="windowFrame" 
 
        BorderBrush="#000" 
        Background="#000" 
        Opacity="0.40" 
 
        BorderThickness="1" 
        CornerRadius="5,5,0,0" Margin="0,0,0,0" Grid.ColumnSpan="2" >
        </Border>
        <Button Content="Ок" HorizontalAlignment="Left" Click="Ok_Click" Margin="46,215,0,0" VerticalAlignment="Top" Width="162" Height="35"/>
        <Button Content="Закрыть" HorizontalAlignment="Left" Click="Close_Click" Margin="46,385,0,0" VerticalAlignment="Top" Width="162" Height="28"/>
        <TextBox HorizontalAlignment="Left" Height="50" Margin="46,60,0,0" TextWrapping="Wrap"  VerticalAlignment="Top" Width="162"/>


    </Grid>
</Window>

<Window x:Class="WpfApp1.Window1" 
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:WpfApp1" 
mc:Ignorable="d" 

 Title="MainWindow" Height="700" Width="450" AllowsTransparency="True" Background="Transparent" WindowStyle="None" ResizeMode="CanResizeWithGrip" Topmost="True">
    <Grid Margin="-23,-31,0,0">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="0*"/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Border Width="Auto" Height="Auto" Name="windowFrame" 

BorderBrush="#395984" 
Background="#002342" 
Opacity="0.5" 

BorderThickness="10" 
CornerRadius="10,10,10,10" Margin="28,37,10,10" Grid.ColumnSpan="2" >
        </Border>




        <Button x:Name="closeButton" Click="closeButton_Click" Content="Close" Margin="156,543,122,135" Grid.ColumnSpan="2" />

        <Border x:Name="border1" BorderBrush="Black" BorderThickness="1" HorizontalAlignment="Left" Height="10" Background="#fff" VerticalAlignment="Top" Width="338" Margin="56,643,0,0" Grid.ColumnSpan="2"/>
        <MediaElement Grid.ColumnSpan="2" Source="" HorizontalAlignment="Left" Height="151" Margin="90,106,0,0" VerticalAlignment="Top" Width="304"/>


    </Grid>
</Window>

public double screenHeight = SystemParameters.FullPrimaryScreenHeight;
        public double screenWidth = SystemParameters.FullPrimaryScreenWidth;
        this.Top = (screenHeight - transparentWindows.Height) / 0x00000002;
        this.Left = (screenWidth - transparentWindows.Width) / 0x00000002;