window sollte diese größe haben Height="450" Width="300" und der background muss geändert werden
 
 <Window.Resources>
     <Style TargetType="Button">
         <Setter Property="Background" Value="White"/>
         <Setter Property="Margin" Value="1"/>
         <Setter Property="FontSize" Value="22"/>
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type Button}">
                     <Border Background="{TemplateBinding Background}" BorderBrush="Transparent" BorderThickness="1">
                         <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
                     </Border>
                 </ControlTemplate>
             </Setter.Value>
         </Setter>
         <Style.Triggers>
             <Trigger Property="IsMouseOver" Value="True">
                 <Setter Property="Background" Value="LightGray"/>
             </Trigger>
             <Trigger Property="IsPressed" Value="True">
                 <Setter Property="Background" Value="WhiteSmoke"/>
             </Trigger>
         </Style.Triggers>
     </Style>
     <Style TargetType="TextBox">
         <Setter Property="HorizontalContentAlignment" Value="Right"/>
         <Setter Property="VerticalContentAlignment" Value="Bottom"/>
         <Setter Property="IsReadOnly" Value="True"/>
         <Setter Property="IsHitTestVisible" Value="False"/>
         <Setter Property="BorderBrush" Value="Transparent"/>
         <Setter Property="Background" Value="White"/>
     </Style>
 </Window.Resources>
 <Grid>
     <Grid.RowDefinitions>
         <RowDefinition Height="*"/>
         <RowDefinition Height="3*"/>
     </Grid.RowDefinitions>
     <DockPanel>
         <TextBox DockPanel.Dock="Top" x:Name="TextBox_Rechenweg"/>
         <TextBox x:Name="TextBox_Ausgabe" FontSize="48"/>
     </DockPanel>
     <DockPanel Grid.Row="1">
         <Grid>
             <Grid.ColumnDefinitions>
                 <ColumnDefinition Width="*"/>
                 <ColumnDefinition Width="*"/>
                 <ColumnDefinition Width="*"/>
                 <ColumnDefinition Width="*"/>
                 <ColumnDefinition Width="*"/>
             </Grid.ColumnDefinitions>
             <Grid.RowDefinitions>
                 <RowDefinition Height="*"/>
                 <RowDefinition Height="*"/>
                 <RowDefinition Height="*"/>
                 <RowDefinition Height="*"/>
                 <RowDefinition Height="*"/>
             </Grid.RowDefinitions>
             <Button Content="C" Grid.ColumnSpan="3" Click="Button_Löschen_Click"/>
             <Button Content="±" Grid.Column="3" Click="Button_PlusMinus_Click"/>
             <Button Content="Sqrt" Grid.Column="4" Click="Button_Operator_Click"/>
             <Button Content="7" Grid.Row="1" Click="Button_Eingabe_Click"/>
             <Button Content="8" Grid.Row="1" Grid.Column="1" Click="Button_Eingabe_Click"/>
             <Button Content="9" Grid.Row="1" Grid.Column="2" Click="Button_Eingabe_Click"/>
             <Button Content="/" Grid.Row="1" Grid.Column="3" Click="Button_Operator_Click"/>
             <Button Content="%" Grid.Row="1" Grid.Column="4" Click="Button_Operator_Click"/>
             <Button Content="4" Grid.Row="2" Click="Button_Eingabe_Click"/>
             <Button Content="5" Grid.Row="2" Grid.Column="1" Click="Button_Eingabe_Click"/>
             <Button Content="6" Grid.Row="2" Grid.Column="2" Click="Button_Eingabe_Click"/>
             <Button Content="*" Grid.Row="2" Grid.Column="3" Click="Button_Operator_Click"/>
             <Button Content="Pow" Grid.Row="2" Grid.Column="4" Click="Button_Operator_Click"/>
             <Button Content="1" Grid.Row="3" Click="Button_Eingabe_Click"/>
             <Button Content="2" Grid.Row="3" Grid.Column="1" Click="Button_Eingabe_Click"/>
             <Button Content="3" Grid.Row="3" Grid.Column="2" Click="Button_Eingabe_Click"/>
             <Button Content="-" Grid.Row="3" Grid.Column="3" Click="Button_Operator_Click"/>
             <Button Content="=" Grid.Row="3" Grid.Column="4" Grid.RowSpan="2" Click="Button_Gleich_Click"/>
             <Button Content="0" Grid.Row="4" Grid.ColumnSpan="2" Click="Button_Eingabe_Click"/>
             <Button Content="," Grid.Row="4" Grid.Column="2" Click="Button_Eingabe_Click"/>
             <Button Content="+" Grid.Row="4" Grid.Column="3"/>
         </Grid>
     </DockPanel>
 </Grid>