What I meant by "You would better work with the underlying data elements." is that if your ItemsSource is a collection of YourObject objects, then you should work with the YourObject business object associated with a particular cell/row.
You can still acess the Cells for the GridViewRow using the ChildrenOfType extension method, but please note that this will return only the visible cells if you use the UI Virtualization that is available by default. To get all the cells, then you should disable the virtualization (setting EnableRowVirualization="False" and EnableColumnVirtualization="False" as explained in the help article). Then you could get all the cells and work with the individual Cell to assign a Style for it. I hope this will work for you better.
the Telerik team