SignUpWindow.xaml

<Window x:Class="WPFteste.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:WPFteste"
        mc:Ignorable="d"
        Title="Window1" Height="450" Width="400"
        WindowStartupLocation="CenterScreen">

    <Border>
        <Border.Background>
            <LinearGradientBrush StartPoint="0,1" EndPoint="1,1">
                <GradientStop Color="Black"/>
            </LinearGradientBrush>
        </Border.Background>
        <Grid>
            <Label Content="Usuário" HorizontalAlignment="Left" Margin="30,57,0,0" VerticalAlignment="Top" Width="77" FontSize="20" Foreground="White" FontFamily="Adobe Hebrew"/>
            <TextBox x:Name="Textbox_user" HorizontalAlignment="Left" Margin="30,99,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="300" Height="38" FontSize="20"/>
            <Label Content="Senha" HorizontalAlignment="Left" Margin="30,168,0,0" VerticalAlignment="Top" Width="64" FontSize="20" Foreground="White"/>
            <TextBox x:Name="Textbox_senha" HorizontalAlignment="Left" Margin="30,210,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="300" Height="38" FontSize="20" TextChanged="TextBox_TextChanged"/>
            <Button Content="Registrar" Click="Button_Click" Margin="30,341,30,37" FontSize="20"/>
        </Grid>
    </Border>
</Window>