How to add watermark to ComboBox

I would like to apply a watermark above ComboBoxes just like the TextBox control includes.  First, it would be great if that option was added to the control.  Since that's not availble today, I need to maually add it myself.  The following code is producing the desired results on my screen, however, I would like to apply the same style and margins generated by the Actipro TextBox control to display the watermark.

<TextBlock
	Classes="theme-form-control-label"
	Margin="0,8,2,2"
	Text="{x:Static lib:Resources.MISC_OPTIONS_THEME_LABEL}" />

<ComboBox
	Classes="theme-outline w400 mb-1"
	ItemsSource="{Binding AvailableThemeTypes}"
	SelectedItem="{Binding SelectedTheme}"
	SelectionChanged="Theme_SelectionChanged">
	<ComboBox.ItemTemplate>
		<DataTemplate>
			<TextBlock Text="{Binding Name}" />
		</DataTemplate>
	</ComboBox.ItemTemplate>
</ComboBox>