Here's how one might create the converter:

    public class ThemeStyleConverter : IValueConverter
    {
        public Style Style
        {
            get;
            set;
        }

        public Type ElementType
        {
            get;
            set;
        }

        object IValueConverter.Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var strValue = value as string;

            if (strValue != null && this.ElementType != null)
            {
                var rs = ThemeManager.GetResourceSet(strValue, ThemeManager.AllGroupingsLiteral);

                if (rs != null)
                {
                    var style = rs[this.ElementType] as Style;

                    if (null != style)
                    {
                        // since we're binding if a Style wasn't provided we have to return the one 
                        // we find or the property will be set to null which is a local value and 
                        // will be the implicit style
                        if (this.Style == null)
                            return style;

                        Debug.Assert(this.Style.BasedOn == null);

                        var newStyle = new Style(this.ElementType);
                        newStyle.BasedOn = style;

                        foreach (var setter in this.Style.Setters)
                            newStyle.Setters.Add(setter);

                        foreach (var trigger in this.Style.Triggers)
                            newStyle.Triggers.Add(trigger);

                        // note it might be possible to just set the BasedOn but if the Style
                        // was sealed then this won't be allowed so i'm just creating a new one
                        return newStyle;
                    }
                }
            }

            return this.Style;
        }

        object IValueConverter.ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return DependencyProperty.UnsetValue;
        }
    }

And then one might use it like so:

        <igWindows:XamTabControl Grid.Row="1" Theme="{Binding ElementName=cboTheme, Path=SelectedItem}">
            <igWindows:XamTabControl.ItemContainerStyle>
                <Binding Path="Theme" RelativeSource="{RelativeSource Self}">
                    <Binding.Converter>
                        <local:ThemeStyleConverter ElementType="{x:Type igWindows:TabItemEx}">
                            <local:ThemeStyleConverter.Style>
                                <Style TargetType="igWindows:TabItemEx">
                                    <Style.Triggers>
                                        <DataTrigger Binding="{Binding Path=Tag, RelativeSource={RelativeSource Self}}" Value="{x:Null}" >
                                            <Setter Property="FontStyle" Value="Italic"/>
                                            <Setter Property="FontFamily" Value="Arial" />
                                        </DataTrigger>
                                    </Style.Triggers>
                                </Style>
                            </local:ThemeStyleConverter.Style>
                        </local:ThemeStyleConverter>
                    </Binding.Converter>
                </Binding>
            </igWindows:XamTabControl.ItemContainerStyle>
            <igWindows:TabItemEx Header="Tab 1"/>
            <igWindows:TabItemEx Header="Tab 2" />
            <igWindows:TabItemEx Header="Tab 3" />
        </igWindows:XamTabControl>

So basically in this case I'm just binding to the Theme of the XamTabControl instance. The converter for the binding takes the Style that needs to be merged with the Theme's Style and the ElementType defines the Type for which the element should be obtained. One might be able to remove the latter assuming one could rely on the TargetType of the Style that is provided to the converter being set but I prefer to explicitly provide it.

Hello,

Thanks for trying but as soon as we then set a style on the each of the tab items localy to set the header property then it no longer works.

Andrew.

I never said that would allow you to set the Style on the TabItem. That would mean the ItemContainerStyle would not be used. There is only one local Style for an element. The binding with the converter would still work. You would just need to do that on every tab. Personally I think a per element Style to set a Header is overkill when one can just bind the Header even using a multibinding if you need to obtain information from multiple properties.

Thanks for you help.

Andrew.