Set slider DefaultForeground value in style

This fixes potential issues with shared instances of the brush

Author: robloo

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPickerSlider.Properties.cs

@@ -104,7 +104,7 @@ public ColorRepresentation ColorRepresentation
                 typeof(Brush),
                 typeof(ColorPickerSlider),
                 new PropertyMetadata(
-                    new SolidColorBrush(Colors.Gray),
+                    null,
                     (s, e) => (s as ColorPickerSlider)?.OnDependencyPropertyChanged(s, e)));
 
         /// <summary>

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPickerSlider.xaml

@@ -9,6 +9,7 @@
            TargetType="controls:ColorPickerSlider">
         <Setter Property="VerticalAlignment" Value="Center" />
         <Setter Property="IsThumbToolTipEnabled" Value="False" />
+        <Setter Property="DefaultForeground" Value="Gray" />
         <Setter Property="Template">
             <Setter.Value>
                 <!--  Based on WinUI version 2.4.2  -->