TreeViewItem selected background color not updating

I have set the theme like so where color700string is some random color:

ThemeManager.BeginUpdate();
            try
            {
                ThemeManager.RegisterThemeDefinition(new ThemeDefinition("Custom")
                {
                    BaseColorBlue = (System.Windows.Media.Color)System.Windows.Media.ColorConverter.ConvertFromString(color700String)
                });

                ThemeManager.AreNativeThemesEnabled = true;
                ThemeManager.CurrentTheme = "Custom";
            }
            finally
            {
                ThemeManager.EndUpdate();
            }

I have set the style like so:

    <Style TargetType="{x:Type TreeViewItem}">
        <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}"/>
        <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
        <Setter Property="local:FocusExtension.IsFocused" Value="{Binding IsFocused, Mode=TwoWay}"/>
        <Setter Property="HorizontalContentAlignment" Value="Left" />
        <Setter Property="VerticalContentAlignment" Value="Center" />      
    </Style>

The background selected color for TreeViewItems found in TreeViews is always the system color.