Hi Martin,
Thanks for contacting Syncfusion support.
We have analyzed your query .You can acheive this requirement by applying custom style for the GridCell. By editing the "PART_CurrentCellBorder" part of GridCell style, you can apply style for current cell. We have created simple sample to acheive this requirment, which can be downloaded under following location,
Sample: https://s3.amazonaws.com/files2.syncfusion.com/dtsupport/directtrac/110164/CustomSample-1595148795.zip
Note: In this sample we have modified the background color of current cell.
Please let us know if you need further assistance.
Regards,
Riyaj Ahamed I