fixed #552

@cuiliang 单独就不控制了，这是主题的事，颜色改深了

src/Shared/HandyControl_Shared/Themes/Styles/Base/ScrollViewerBaseStyle.xaml

@@ -147,7 +147,7 @@
     <Style x:Key="ScrollBarBaseStyle" TargetType="ScrollBar">
         <Setter Property="Stylus.IsPressAndHoldEnabled" Value="false"/>
         <Setter Property="Stylus.IsFlicksEnabled" Value="false"/>
-        <Setter Property="Foreground" Value="{DynamicResource ThirdlyTextBrush}"/>
+        <Setter Property="Foreground" Value="{DynamicResource PrimaryTextBrush}"/>
         <Setter Property="Background" Value="Transparent"/>
         <Setter Property="Focusable" Value="False"/>
         <Setter Property="BorderThickness" Value="0"/>
@@ -304,4 +304,4 @@
         <Setter Property="Template" Value="{StaticResource ScrollViewerBaseControlTemplate}"/>
     </Style>
 
-</ResourceDictionary>
+</ResourceDictionary>