Here is a simple tip to add a custom background for the Comment boxes..
Proceed as follow:
Select a Cell/Right Click
Select the "Insert Comment" option
A red triangle appears at the top right of the cell
Delete the text if you want
Click on the hatched frame and then right click: Format Comment
Select Color and Lines tab/Fill/Color/click on the small arrow to expand the options.
Select the "Fill Effets Options" and in the new Window that opens Select the Texture Tab.
Click on the "Other texture" button and select an image from your hard drive (or any other media)
E.g: With a Custom image:
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