enhance: Optimized the ScrollViewer style.

src/Net_40/HandyControl_Net_40/Themes/Theme.xaml

@@ -2145,10 +2145,9 @@
 		<Style.Triggers>
 			<Trigger Property="Orientation" Value="Horizontal">
 				<Setter Property="Width" Value="Auto" />
-				<Setter Property="MinWidth" Value="0" />
+				<Setter Property="MinWidth" Value="8" />
 				<Setter Property="Height" Value="8" />
 				<Setter Property="MinHeight" Value="8" />
-				<Setter Property="BorderThickness" Value="0,1" />
 				<Setter Property="Template">
 					<Setter.Value>
 						<ControlTemplate TargetType="ScrollBar">