Provide a style property for the text in grouping panel. Currently you can style the border, background color and change the header text of the panel. For example:
<Style TargetType="telerik:DataGridGroupingPanel">
<Setter Property="BackgroundColor" Value="Red"/>
<Setter Property="HeaderText" Value="Hello to group"/>
<Setter Property="CornerRadius" Value="10"/>
<Setter Property="BorderThickness" Value="2"/>
<Setter Property="BorderColor" Value="Blue"/>
</Style>