Change xaml property from Color to Brush to prevent crash

Author: tommy9

Rubberduck.Core/UI/Controls/ToolBar.xaml

@@ -458,7 +458,7 @@
         <Setter Property="Foreground" Value="{Binding IsDimmed, Mode=OneWay, Converter={StaticResource BooleanToDimmed}}" />
         <Style.Triggers>
             <DataTrigger Binding="{Binding IsObsolete}" Value="True">
-                <Setter Property="Foreground" Value="{StaticResource CaptionDisabledColor}" />
+                <Setter Property="Foreground" Value="{StaticResource CaptionDisabledBrush}" />
                 <Setter Property="TextDecorations" Value="Strikethrough" />
             </DataTrigger>
         </Style.Triggers>