Metro Style WPF window Sample

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        Height="350"
        Width="525"
        Background="Gray"
        AllowsTransparency="True"
        WindowStyle="None"
        MouseLeftButtonDown="Window_MouseLeftButtonDown"
        WindowStartupLocation="CenterScreen"
        ResizeMode="CanMinimize">
    <Window.Resources>
        <Style x:Key="TransparentButtonStyle"
               TargetType="{x:Type Button}">
            <Setter Property="Background" Value="Transparent"/>
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="BorderBrush" Value="Transparent"/>
            <Setter Property="BorderThickness" Value="0"/>
            <Setter Property="HorizontalContentAlignment" Value="Center"/>
            <Setter Property="VerticalContentAlignment" Value="Center"/>
            <Setter Property="Padding" Value="10 5"/>
            <Setter Property="FontSize" Value="12" />
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Button}">
                        <Grid>
                            <Border x:Name="Border"
                                Background="{TemplateBinding Background}"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="{TemplateBinding BorderThickness}" />

                            <ContentPresenter x:Name="content"
                                HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                Margin="{TemplateBinding Padding}"
                                VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                RecognizesAccessKey="True" />
                        </Grid>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsPressed" Value="True">
                            </Trigger>
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter Property="BorderThickness" Value="0"/>
                                <Setter Property="Background" Value="#CCCCCC" TargetName="Border"/>
                                <Setter Property="Opacity" Value="1" TargetName="Border"/>
                                <Setter Property="Foreground" Value="Black"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Foreground" Value="#ADADAD"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <Button x:Name="CLOSE_BUTTON"
                Style="{StaticResource TransparentButtonStyle}"
                VerticalAlignment="Top"
                HorizontalAlignment="Right"
                Background="Transparent"
                Height="30"
                Width="30"
                Click="Close_Click">
        <Path Data="F1M54.0573,47.8776L38.1771,31.9974 54.0547,16.1198C55.7604,14.4141 55.7604,11.6511 54.0573,9.94531 52.3516,8.23962 49.5859,8.23962 47.8802,9.94531L32.0026,25.8229 16.1224,9.94531C14.4167,8.23962 11.6511,8.23962 9.94794,9.94531 8.24219,11.6511 8.24219,14.4141 9.94794,16.1198L25.8255,32 9.94794,47.8776C8.24219,49.5834 8.24219,52.3477 9.94794,54.0534 11.6511,55.7572 14.4167,55.7585 16.1224,54.0534L32.0026,38.1745 47.8802,54.0534C49.5859,55.7585 52.3516,55.7572 54.0573,54.0534 55.7604,52.3477 55.763,49.5834 54.0573,47.8776z"
              Stretch="Uniform"
              Fill="Red"
              Width="10"
              Margin="0,0,0,0">
        </Path>
        </Button>

    </Grid>
</Window>

.cs file

        private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            this.DragMove();
        }
       
        private void Close_Click(object sender, RoutedEventArgs e)
        {
            this.Close();
        }

Output: