feat: add avalonia EmbeddableControlRoot style.

Author: NaBian

src/Avalonia/HandyControl_Avalonia/Themes/Styles/EmbeddableControlRoot.axaml

@@ -0,0 +1,31 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+    <ControlTheme x:Key="{x:Type EmbeddableControlRoot}"
+                  TargetType="EmbeddableControlRoot">
+        <Setter Property="Foreground"
+                Value="{DynamicResource PrimaryTextBrush}" />
+        <Setter Property="Background"
+                Value="{DynamicResource RegionBrush}" />
+        <Setter Property="FontSize"
+                Value="12" />
+        <Setter Property="Template">
+            <ControlTemplate>
+                <Panel>
+                    <Border Name="PART_TransparencyFallback"
+                            IsHitTestVisible="False" />
+                    <Border Background="{TemplateBinding Background}">
+                        <VisualLayerManager>
+                            <ContentPresenter Name="PART_ContentPresenter"
+                                              Margin="{TemplateBinding Padding}"
+                                              Content="{TemplateBinding Content}"
+                                              ContentTemplate="{TemplateBinding ContentTemplate}" />
+                        </VisualLayerManager>
+                    </Border>
+                </Panel>
+            </ControlTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -19,6 +19,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/DataValidationErrors.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/FlyoutPresenter.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/MenuFlyoutPresenter.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/EmbeddableControlRoot.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/TransitioningContentControl.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PopupRoot.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PathIcon.axaml" />