WPF SyntaxEditor in ElementHost fails to change scrollbar theme

If you create a WPF app and add a WPF SyntaxEditor control, it sets the colours as per the current theme as expected.

However, if you create a WinForms app and add a WPF SyntaxEditor as the child of the ElementHost control, all theme aspects are correct appart from the scrollbars.

1. Create a WinForms application called 'WindowsFormsApp1' and references for ActiproSoftware.Shared / SyntaxEditor / Text WPF dlls.
2. Add the file from the Actipro Samples 'Dark.vssettings' as an embedded resource
3. Copy the following code into 'WindowsFormsApp1.cs'
4. Run the application, and you should see the MetroDark theme is used for everything except the scrollbars?

using ActiproSoftware.Windows.Controls.SyntaxEditor;
using ActiproSoftware.Windows.Controls.SyntaxEditor.Highlighting;
using ActiproSoftware.Windows.Controls.SyntaxEditor.IntelliPrompt.Implementation;
using ActiproSoftware.Windows.Themes;
using System;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Windows.Forms;
using System.Windows.Forms.Integration;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        private ElementHost elementHost;
        private SyntaxEditor syntaxEditor;

        public Form1()
        {
            InitializeComponent();

            elementHost = new ElementHost();
            this.Controls.Add(elementHost);
            elementHost.Dock = DockStyle.Fill;
            syntaxEditor = new SyntaxEditor();
            elementHost.Child = syntaxEditor;
        }

        protected override void OnLoad(EventArgs e)
        {
            ThemeManager.BeginUpdate();
            try
            {
                ThemeManager.AreNativeThemesEnabled = true;
                ThemeManager.CurrentTheme = ThemeName.MetroDark.ToString();
            }
            finally
            {
                ThemeManager.EndUpdate();
            }

            var classificationTypes = AmbientHighlightingStyleRegistry.Instance.ClassificationTypes.ToArray();
            foreach (var classificationType in classificationTypes)
                AmbientHighlightingStyleRegistry.Instance.Unregister(classificationType);

            new DisplayItemClassificationTypeProvider().RegisterAll();

            using (Stream stream = Assembly.GetExecutingAssembly().GetManifestResourceStream("WindowsFormsApp1.Dark.vssettings"))
            {
                AmbientHighlightingStyleRegistry.Instance.ImportHighlightingStyles(stream);
            }

            CommonImageSourceProvider.DefaultImageSet = CommonImageSet.MetroDark;

            base.OnLoad(e);
        }
    }
}