You can achieve that by editing the control template for GridViewToggleButtonTemplate, create a style and use the ToggleButtonStyle property of the GridViewToggleRowDetailsColumn to apply the style.
Attached, you can find a sample project in which I've demonstrated this approach, replacing the +/- signs with RadGlyph but you can configure it to your liking. Please, review the project and let me know if it delivered the desired result.
Regards,
Dimitar Dinev
Progress Telerik
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