Thank you for writing back.
The element the arrows point at is the table element of the grid details row cell. You can use the ViewCellFormatting event to change this color. Here is a code snippet which demonstrates how:
sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
GridDetailViewCellElement cell = e.CellElement
cell.ChildTableElement.BackColor = Color.Gray;
Again, as Anton mentioned in his first post, changing this color will change the colors of the rows/cells as they are transparent by default.
I am not sure I fully understand your second question. The best way to customize cell elements is through the CellFormatting/ViewCellFormating events. There you have access to the actual element that will be painted and you can modify it. The formatting events are also fired for only the visible cells as they are the only elements created (thanks to the grid UI virtualization) so they are not slowing down the grid. If this does not answer your questions, I would kindly ask you to provide some more information on what you want to achieve.
I hope this will be useful. Should you have further questions, do not hesitate to write back.
the Telerik team