ウィンドウ（フォーム）背景に模様

App.xaml

<Application
    x:Class="SampleApp.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:SampleApp"
    StartupUri="MainWindow.xaml">
    <Application.Resources>

        <!--  既定のButtonスタイル  -->
        <Style TargetType="Button">
            <Setter Property="Margin" Value="8" />
            <Setter Property="Width" Value="150" />
            <Setter Property="HorizontalAlignment" Value="left" />
        </Style>

        <!--  模様付きBorderスタイル  -->
        <Style x:Key="CheckeredPatternStyle" TargetType="Border">
            <Setter Property="Background">
                <Setter.Value>
                    <DrawingBrush
                        Stretch="None"
                        TileMode="Tile"
                        Viewport="0,0,100,100"
                        ViewportUnits="Absolute">
                        <DrawingBrush.Drawing>
                            <DrawingGroup>
                                <GeometryDrawing Brush="Black">
                                    <GeometryDrawing.Geometry>
                                        <RectangleGeometry Rect="0,0,100,100" />
                                    </GeometryDrawing.Geometry>
                                </GeometryDrawing>
                                <GeometryDrawing Brush="ForestGreen">
                                    <GeometryDrawing.Geometry>
                                        <GeometryGroup RenderOptions.EdgeMode="Aliased">
                                            <RectangleGeometry Rect="0,0,50,50" />
                                            <RectangleGeometry Rect="50,50,50,50" />
                                        </GeometryGroup>
                                    </GeometryDrawing.Geometry>
                                </GeometryDrawing>
                            </DrawingGroup>
                        </DrawingBrush.Drawing>
                    </DrawingBrush>
                </Setter.Value>
            </Setter>
        </Style>

    </Application.Resources>
</Application>

MainWindow.xaml

<Window
    x:Class="SampleApp.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:local="clr-namespace:SampleApp"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="MainWindow"
    Width="600"
    Height="400"
    MinWidth="250"
    MinHeight="200"
    ResizeMode="CanResizeWithGrip"
    mc:Ignorable="d">

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition Width="0.4*" />
        </Grid.ColumnDefinitions>

        <Border Grid.ColumnSpan="2" Style="{StaticResource CheckeredPatternStyle}" />

        <StackPanel>
            <Button Content="Button" />
            <Button Content="Button" />
            <Button Content="Button" />
        </StackPanel>

        <TextBox
            Grid.Column="1"
            Margin="8"
            Opacity="0.9"
            Text="こんにちは"
            VerticalScrollBarVisibility="Visible" />
    </Grid>
</Window>

csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net5.0-windows</TargetFramework>
    <UseWPF>true</UseWPF>
  </PropertyGroup>

</Project>