Grouping items disregards items style ( Groups show up empty )

Answer 1

I'm not sure what's causing your problems based on the information you provided above. But the code below, which is an update to the answer I gave on the link you specified, works fine for me:

Styles:

<Style x:Key="ComboBoxItemStyle" TargetType="ComboBoxItem">
    <Setter Property="Foreground" Value="Red"/>
</Style>

<Style x:Key="ComboBoxStyle" BasedOn="{StaticResource {x:Type ComboBox}}" TargetType="{x:Type ComboBox}">
    <Setter Property="ItemContainerStyle" Value="{StaticResource ComboBoxItemStyle}"/>
</Style>

ComboBox:

<ComboBox x:Name="comboBox" Style="{StaticResource ComboBoxStyle}">
    <ComboBox.GroupStyle>
        <GroupStyle>
            <GroupStyle.HeaderTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
            </GroupStyle.HeaderTemplate>
        </GroupStyle>
    </ComboBox.GroupStyle>
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}"/>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

Hope this will help you solve your problem.