There is no way to change the cell styles of a grid. It is only possible to change the text styles, but not the whole cell.
There is a way to change the whole cell style in the 2.x version as explained in:
You should use ColumnData class to modify the styles of the grid. This class exitsts in GXT 3.x but it is not accessible from the renderer. It is to say, the operation "public void render(Cell.Context context, C value, SafeHtmlBuilder sb)" of the class "PropertyDisplayCell<C>" cant access any instance of the class "ColumnData".
Finally, is it not possible to change the styles of the whole cell as you only have access to the inner text. The only way to change the text style is to do something like:
public void render( context, T value, SafeHtmlBuilder safeHtmlBuilder) {
   safeHtmlBuilder.appendHtmlConstant("<span style='"+customStyle+"'>");
   super.render(context, value, safeHtmlBuilder);
But, as said, it is not possible to change the whole cell style.
In fact there is no usage of the class ColumnData: