First, I would suggest you configuring RadGridView with GroupRenderMode to "Flat".
The Flat mode
represents a completely new logic for group rendering, which fixed a lot of other issues reproducible with the old Nested mode. In that case, you will need to work with GroupHeaderRow and not GridViewGroupRow.
You can then check the Styling the Group Row topic in our online documentation on how to style the group rows differently.
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