WPF control not displaying in ElementHost in WinForms app

I have a problem with a WPF control that I'm trying to host in an ElementHost in a WinForms app. The control is a lookless custom control that I originally developed in a separate test project, which was a WPF app. In there it obviously works fine, but in my WinForms app all I get is a blank grey box where the ElementHost is displayed.

Here's my C# code for creating, populating, and adding the ElementHost to the parent Control:

// This is my WPF control
m_TabHostPanel  = new TabHostPanel();
m_ElementHost  = new ElementHost
                 {
                     Child = m_TabHostPanel,
                     Dock = DockStyle.Top,
                     Height = 34
                 };
this.Controls.Add( m_ElementHost );

The parent control contains other WinForms controls that are added and removed at runtime, as needed. These are all hosted singly with their Dock set to DockStyle.Fill. Thus, every time I add one I send the ElementHost to the back of the Z-order to make sure it renders correctly:

m_ElementHost.SendToBack();

Thus, I know I'm not running into an airspace problem, or anything like that.

The one thing I did wonder about is this: in the original project the styles for all my lookless controls were merged into the resource dictionary for the application in App.xaml, like this:

<Application x:Class="WpfTestApp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Application/UserInterface/DataTemplates/TabModelDataTemplate.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/HoverablePressableButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/MiniControlButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabCloseButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabScrollLeftButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabScrollRightButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabListDropDownButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabHostComboBoxStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabButtonStyle.xaml"/>
                <ResourceDictionary Source="Application/UserInterface/Styles/TabHostPanelStyle.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

I've migrated App.xaml to my WinForms project, but the build action is set to Page. If I set it to ApplicationDefinition I get an error saying that the application has multiple entry points, which makes sense, but I'm wondering then if the styles, etc., are being picked up. If not this might explain why I'm getting a blank grey rectangle where my control should be because, without these, there's nothing to define its look. So maybe the question is, how do I get these styles into my WinForms application so that my WPF controls can see them?

I should probably also mention that this is running on .NET Fx 3.5.

Anyway, for now I'm perplexed, so any help would be gratefully received.

Many thanks!

Bart