You can achieve the desired result by creating a style targeting the GroupHeaderRow if you're using the Flat GroupRenderMode or the GridViewGroupRow if you're using the Nested mode. As we recommend using the Flat mode, I'm providing the required style when using this setting.
I've prepared a small sample project to demonstrate both scenarios.
Please have a look and let me know if you're able to achieve the desired result.
Regards,
Dilyan Traykov
Progress Telerik
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