I'm not sure how you specified the custom template, but you can customize the name cells in the same manner as the value cells. Therefore, you can add the following code to your PropertyGrid:
<propgrid:PropertyGrid.PropertyEditors>
<propgrid:PropertyEditor>
<propgrid:PropertyEditor.NameTemplate>
<DataTemplate>
<TextBlock Margin="4,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Center"
TextTrimming="CharacterEllipsis"
Text="{Binding DisplayName, RelativeSource={RelativeSource AncestorType={x:Type propgrid:IPropertyDataAccessor}}}" />
</DataTemplate>
</propgrid:PropertyEditor.NameTemplate>
</propgrid:PropertyEditor>
</propgrid:PropertyGrid.PropertyEditors>
This overrides the default DataTemplate used by the PropertyGrid for all properties. You could also target specific properties or property types by setting the PropertyName or PropertyType on the PropertyEditor element.
Hope this helps.