Unfortunately the functionality described is not implemented. Thank you for suggesting it. I will discuss this request with my colleagues and we might include it in a future release.
As a workaround you may change the SimpleCellContainer and the SimpleTreeCellContainer styles. I've attached a project that adds a red border of the grid cells. Try modifying the styles and the ControlTemplates of the SimpleCellContainer and the SimpleTreeCellContainer to achieve the desired effect. In the project I am using the Office_Black theme so change it with the one you are using.
Hopefully this helps. Feel free to contact us in case you have any problems or concerns.
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