Avalonia/src/Avalonia.Themes.Simple/Controls/WindowDrawnDecorations.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">

  <x:Double x:Key="CaptionButtonWidth">45</x:Double>
  <x:Double x:Key="CaptionButtonHeight">30</x:Double>

  <ControlTheme x:Key="SimpleDrawnCaptionButton" TargetType="Button">
    <Setter Property="Background" Value="{DynamicResource CaptionButtonBackground}" />
    <Setter Property="BorderBrush" Value="{DynamicResource CaptionButtonBorderBrush}" />
    <Setter Property="Foreground" Value="{DynamicResource CaptionButtonForeground}"/>
    <Setter Property="Width" Value="{DynamicResource CaptionButtonWidth}"/>
    <Setter Property="Height" Value="{DynamicResource CaptionButtonHeight}"/>
    <Setter Property="VerticalAlignment" Value="Stretch"/>
    <Setter Property="(WindowDecorationProperties.ElementRole)" Value="DecorationsElement"/>
    <Setter Property="Template">
      <ControlTemplate>
        <ContentPresenter Name="PART_ContentPresenter"
                          Background="Transparent"
                          Content="{TemplateBinding Content}"/>
      </ControlTemplate>
    </Setter>

    <Style Selector="^:pointerover /template/ ContentPresenter">
      <Setter Property="Background" Value="{TemplateBinding Background}" />
    </Style>

    <Style Selector="^:pressed /template/ ContentPresenter">
      <Setter Property="Background" Value="{TemplateBinding BorderBrush}" />
    </Style>
  </ControlTheme>

  <ControlTheme x:Key="{x:Type WindowDrawnDecorations}" TargetType="WindowDrawnDecorations">
    <Setter Property="DefaultTitleBarHeight" Value="30"/>
    <Setter Property="DefaultFrameThickness" Value="1"/>
    <Setter Property="DefaultShadowThickness" Value="8"/>
    <Setter Property="Template">
      <WindowDrawnDecorationsTemplate>
        <WindowDrawnDecorationsContent>

          <WindowDrawnDecorationsContent.Underlay>
            <Panel x:Name="PART_UnderlayWrapper">
              <Border x:Name="PART_WindowBorder"
                      Background="{DynamicResource ThemeBackgroundBrush}"
                      BorderThickness="{TemplateBinding FrameThickness}"
                      BorderBrush="{DynamicResource ThemeBorderMidBrush}"
                      IsHitTestVisible="False" />
              <!-- Titlebar: background, title text, and drag area live in underlay -->
              <Panel x:Name="PART_TitleBar" VerticalAlignment="Top"
                     Height="{TemplateBinding TitleBarHeight}"
                     Background="{DynamicResource ThemeBackgroundBrush}"
                     IsVisible="{TemplateBinding HasTitleBar}"
                     WindowDecorationProperties.ElementRole="TitleBar" />
            </Panel>
          </WindowDrawnDecorationsContent.Underlay>

          <WindowDrawnDecorationsContent.Overlay>
            <!-- Overlay: only interactive caption buttons -->
            <Panel x:Name="PART_OverlayWrapper">
              <!-- Title text lives in overlay so it renders above client content -->
              <Panel x:Name="PART_TitleTextPanel" VerticalAlignment="Top"
                     Height="{TemplateBinding TitleBarHeight}"
                     IsHitTestVisible="False"
                     IsVisible="{TemplateBinding HasTitleBar}">
                <TextBlock Text="{TemplateBinding Title}"
                           VerticalAlignment="Center"
                           Margin="12,0,0,0"
                           FontSize="12" />
              </Panel>
              <StackPanel x:Name="PART_OverlayPanel"
                          VerticalAlignment="Top"
                          HorizontalAlignment="Right"
                          Height="{TemplateBinding TitleBarHeight}"
                          Orientation="Horizontal"
                          Spacing="2"
                          IsVisible="{TemplateBinding HasTitleBar}"
                          TextElement.FontSize="10">
                <Button x:Name="PART_FullScreenButton"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="FullScreenButton">
                  <Viewbox Width="11" Margin="2">
                    <Path Name="FullScreenButtonPath"
                          Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M2048 2048v-819h-205v469l-1493 -1493h469v-205h-819v819h205v-469l1493 1493h-469v205h819z" />
                  </Viewbox>
                </Button>
                <Button x:Name="PART_MinimizeButton"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="MinimizeButton"
                        AutomationProperties.Name="Minimize">
                  <Viewbox Width="11" Margin="2">
                    <Path Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M2048 1229v-205h-2048v205h2048z" />
                  </Viewbox>
                </Button>
                <Button x:Name="PART_MaximizeButton"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="MaximizeButton"
                        AutomationProperties.Name="Maximize">
                  <Viewbox Width="11" Margin="2">
                    <Viewbox.RenderTransform>
                      <RotateTransform Angle="-90" />
                    </Viewbox.RenderTransform>
                    <Path Name="RestoreButtonPath"
                          Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M2048 2048v-2048h-2048v2048h2048zM1843 1843h-1638v-1638h1638v1638z"/>
                  </Viewbox>
                </Button>
                <Button x:Name="PART_CloseButton"
                        Background="#ffe81123"
                        BorderBrush="#fff1707a"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="CloseButton"
                        AutomationProperties.Name="Close">
                  <Viewbox Width="11" Margin="2">
                    <Path Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M1169 1024l879 -879l-145 -145l-879 879l-879 -879l-145 145l879 879l-879 879l145 145l879 -879l879 879l145 -145z" />
                  </Viewbox>
                </Button>
              </StackPanel>
            </Panel>
          </WindowDrawnDecorationsContent.Overlay>

          <WindowDrawnDecorationsContent.FullscreenPopover>
            <!-- Shown on hover at top edge in fullscreen mode -->
            <Panel Height="30"
                   VerticalAlignment="Top"
                   Background="{DynamicResource ThemeAccentBrush}"
                   WindowDecorationProperties.ElementRole="TitleBar">
              <TextBlock Text="{TemplateBinding Title}"
                         VerticalAlignment="Center"
                         Foreground="White"
                         Margin="12,0,0,0"
                         FontSize="12" />
              <StackPanel HorizontalAlignment="Right"
                          Orientation="Horizontal"
                          Spacing="2"
                          TextElement.FontSize="10">
                <Button x:Name="PART_PopoverFullScreenButton"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="FullScreenButton">
                  <Viewbox Width="11" Margin="2">
                    <Path Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M205 1024h819v-819h-205v469l-674 -674l-145 145l674 674h-469v205zM1374 1229h469v-205h-819v819h205v-469l674 674l145 -145z" />
                  </Viewbox>
                </Button>
                <Button x:Name="PART_PopoverCloseButton"
                        Background="#ffe81123"
                        BorderBrush="#fff1707a"
                        Theme="{StaticResource SimpleDrawnCaptionButton}"
                        WindowDecorationProperties.ElementRole="CloseButton"
                        AutomationProperties.Name="Close">
                  <Viewbox Width="11" Margin="2">
                    <Path Stretch="UniformToFill"
                          Fill="{DynamicResource CaptionButtonForeground}"
                          Data="M1169 1024l879 -879l-145 -145l-879 879l-879 -879l-145 145l879 879l-879 879l145 145l879 -879l879 879l145 -145z" />
                  </Viewbox>
                </Button>
              </StackPanel>
            </Panel>
          </WindowDrawnDecorationsContent.FullscreenPopover>

        </WindowDrawnDecorationsContent>
      </WindowDrawnDecorationsTemplate>
    </Setter>

    <!-- Shadow: inset border and add drop shadow -->
    <Style Selector="^:has-shadow /template/ Panel#PART_UnderlayWrapper">
      <Setter Property="Margin" Value="{TemplateBinding ShadowThickness}"/>
    </Style>
    <Style Selector="^:has-shadow /template/ Border#PART_WindowBorder">
      <Setter Property="BoxShadow" Value="0 2 10 2 #80000000"/>
    </Style>
    <Style Selector="^:has-shadow /template/ Panel#PART_OverlayWrapper">
      <Setter Property="Margin" Value="{TemplateBinding ShadowThickness}"/>
    </Style>


    <!-- Border: inset titlebar and buttons inside frame -->
    <Style Selector="^:has-border /template/ Panel#PART_TitleTextPanel">
      <Setter Property="Margin" Value="1,1,1,0"/>
    </Style>
    <Style Selector="^:has-border /template/ Panel#PART_TitleBar">
      <Setter Property="Margin" Value="1,1,1,0"/>
    </Style>
    <Style Selector="^:has-border /template/ StackPanel#PART_OverlayPanel">
      <Setter Property="Margin" Value="0,1,1,0"/>
    </Style>

    <!-- Maximized: restore button path changes -->
    <Style Selector="^:maximized /template/ Path#RestoreButtonPath">
      <Setter Property="Data" Value="M2048 410h-410v-410h-1638v1638h410v410h1638v-1638zM1434 1434h-1229v-1229h1229v1229zM1843 1843h-1229v-205h1024v-1024h205v1229z" />
    </Style>

    <!-- Disabled buttons -->
    <Style Selector="^ /template/ Button:disabled">
      <Setter Property="Opacity" Value="0.2"/>
    </Style>

    <!-- Fullscreen: hide overlay and titlebar (popover takes over) -->
    <Style Selector="^:fullscreen /template/ Panel#PART_TitleTextPanel">
      <Setter Property="IsVisible" Value="False" />
    </Style>
    <Style Selector="^:fullscreen /template/ StackPanel#PART_OverlayPanel">
      <Setter Property="IsVisible" Value="False" />
    </Style>
    <Style Selector="^:fullscreen /template/ Panel#PART_TitleBar">
      <Setter Property="IsVisible" Value="False" />
    </Style>
  </ControlTheme>
</ResourceDictionary>