Padding around the content.
By default, this is 12 pixels on the top and 16 pixels on the bottom. This is intended to be combined with children that have 24 pixels of padding on the left and right, and 8 pixels of padding on the top and bottom, so that the content ends up being indented 20 pixels from the title, 24 pixels from the bottom, and 24 pixels from the sides.
The SimpleDialogOption widget uses such padding.
final EdgeInsetsGeometry contentPadding