Add transparency to skin images, fix background color

icons/skins/default/7.svg

icons/skins/default/8.svg

icons/skins/default/9.svg

icons/skins/empty.svg

icons/skins/nature/1.svg

icons/skins/nature/2.svg

icons/skins/nature/3.svg

icons/skins/nature/4.svg

icons/skins/nature/5.svg

icons/skins/nature/6.svg

icons/skins/nature/7.svg

icons/skins/nature/8.svg

icons/skins/nature/9.svg

lib/entities/cell.dart