To add a custom thumbnail to a page firstly you need to add an asset to the pastboard.
Navigate to a asset folder and select an asset
Now either click the cop icon or drag and drop the asset into the temporary select tool.
Now navigate to the folder/page where you'd like the thumbnail applied and click Properties
Now the PASTE option will be visible under the Custom thumbnail option.
Click paste and save your page/folder.
To remove the thumbnail go back into properties and note the remove button is now visible.
