Change Background/Grid of Overview

Our diagram has a grid. We created a simple Overview which resides on top of the diagram when it is visible.
Alas the Overview shows a grid and combined with the diagram grid below it it is very ugly.
We tried to remove the grid from the overview:
overview.grid.visible = false
but it did not work.

If you are OK with no temporary layers being displayed in the overview (including adornments) you can set:

overview.drawsTemporaryLayers = false;