See the example in https://mydemoversion8-sandbox.mxapps.io/p/appdemo/gridcellstyler. The grid on the screen is styled by the Gridcellstyler, which is placed as a widget on the page just below the grid and the image on the examples screen show the settings of that widget.
You can fork it from github (see top right of the example screen) if you want the full project.
There is a nice test project on github. Also a little bit more info on how to configure it.