Update slider to use theme colors.

src/Perspex.Themes.Default/Slider.paml

@@ -16,7 +16,7 @@
               <Thumb.Template>
                 <ControlTemplate>
                   <Grid>
-                    <Ellipse Width="12" Height="12" Fill="Gray"/>
+                    <Ellipse Width="12" Height="12" Fill="{StyleResource ThemeAccentBrush}"/>
                   </Grid>
                 </ControlTemplate>
               </Thumb.Template>
@@ -43,7 +43,7 @@
               <Thumb.Template>
                 <ControlTemplate>
                   <Grid>
-                    <Ellipse Width="12" Height="12" Fill="Gray"/>
+                    <Ellipse Width="12" Height="12" Fill="{StyleResource ThemeAccentBrush}"/>
                   </Grid>
                 </ControlTemplate>
               </Thumb.Template>
@@ -61,6 +61,6 @@
   </Style>
   <Style Selector="Slider /template/ Border#TrackBackground">
     <Setter Property="BorderThickness" Value="2"/>
-    <Setter Property="BorderBrush" Value="LightGray"/>
+    <Setter Property="BorderBrush" Value="{StyleResource ThemeBorderLightBrush}"/>
   </Style>  
 </Styles>