feat: implement CardBorder.

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

@@ -0,0 +1,17 @@
+<UserControl
+    x:Class="Semi.Avalonia.Demo.Pages.BorderDemo" 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 Margin="20" Spacing="20">
+        <Border Theme="{StaticResource CardBorder}">
+            <TextBlock>Card</TextBlock>
+        </Border>
+        <Border Classes="Shadow" Theme="{StaticResource CardBorder}">
+            <TextBlock>Shadow Always Applied</TextBlock>
+        </Border>
+        <Border Classes="Hover" Theme="{StaticResource CardBorder}">
+            <TextBlock>Shadow on Pointerover</TextBlock>
+        </Border>
+    </StackPanel>
+</UserControl>

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

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