add style and demo

src/Shared/HandyControlDemo_Shared/Resources/Themes/Styles/Style.xaml

@@ -1,6 +1,7 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-                    xmlns:hc="https://handyorg.github.io/handycontrol">
+                    xmlns:hc="https://handyorg.github.io/handycontrol"
+                    xmlns:userControl="clr-namespace:HandyControlDemo.UserControl">
 
     <ResourceDictionary.MergedDictionaries>
         <ResourceDictionary Source="../Basic/Geometries.xaml"/>
@@ -98,4 +99,6 @@
         <Setter Property="Padding" Value="25"/>
     </Style>
 
+    <Style BasedOn="{StaticResource ComboBoxItemBaseStyle}" TargetType="userControl:CustomSplitButtonItem"/>
+
 </ResourceDictionary>