Add SplitButtonSeparatorWidth resource

src/Avalonia.Themes.Fluent/Controls/SplitButton.xaml

@@ -23,6 +23,7 @@
   <Styles.Resources>
     <x:Double x:Key="SplitButtonPrimaryButtonSize">32</x:Double>
     <x:Double x:Key="SplitButtonSecondaryButtonSize">32</x:Double>
+    <x:Double x:Key="SplitButtonSeparatorWidth">1</x:Double>
 
     <converters:MarginMultiplierConverter x:Key="PrimaryButtonBorderMultiplier" Left="True" Top="True" Bottom="True" Indent="1" />
     <converters:MarginMultiplierConverter x:Key="SecondaryButtonBorderMultiplier" Right="True" Top="True" Bottom="True" Indent="1" />
@@ -77,7 +78,7 @@
             <Border x:Name="SeparatorBorder"
                     Grid.Column="1"
                     Background="Transparent"
-                    Width="1"
+                    Width="{DynamicResource SplitButtonSeparatorWidth}"
                     BorderThickness="{TemplateBinding BorderThickness, Converter={StaticResource SeparatorBorderMultiplier}}"
                     BorderBrush="{TemplateBinding BorderBrush}" />