feat: WIP: flyout default theme. pending on DynamicResource.

src/Semi.Avalonia.Demo/Pages/FlyoutDemo.axaml

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+<UserControl
+    x:Class="Semi.Avalonia.Demo.Pages.FlyoutDemo"
+    xmlns="https://github.com/avaloniaui"
+    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"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mc:Ignorable="d">
+    <StackPanel>
+        <StackPanel>
+            <Button Content="Hello">
+                <Button.Flyout>
+                    <Flyout Placement="Top" ShowMode="Standard">
+                        <StackPanel>
+                            <TextBlock>Hello World</TextBlock>
+                        </StackPanel>
+                    </Flyout>
+                </Button.Flyout>
+            </Button>
+        </StackPanel>
+    </StackPanel>
+</UserControl>

src/Semi.Avalonia.Demo/Pages/FlyoutDemo.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.LogicalTree;
+using Avalonia.Markup.Xaml;
+
+namespace Semi.Avalonia.Demo.Pages;
+
+public partial class FlyoutDemo : UserControl
+{
+    public FlyoutDemo()
+    {
+        InitializeComponent();
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}