In the Design window, is there any way to hide the grid? I have looked everywhere and cannot see where this is possible.
I guess I found a workaround. I put the horizon very low and at the same time move the item up.
It is not currently possible to turn the grid visualization off, but this is only part of the UI and will not show up in renders. For turning the 'ground' plane effect off you can do that in Design Mode > Environment Properties. Turn off the checkbox next to Ground Plane and you will no longer have shadows/reflections on the floor of the world, giving you a floating appearance.
I recommend posting ideas/suggestions to our Feedback site, https://feedback.adobedimension.com/
just go to View, toggle grid or ⌘"
You can change color of grid from property panel to backgroud color. That can be a solution also.