Add transparency to skin images, fix background color

assets/skins/nature_3.png

assets/skins/nature_4.png

assets/skins/nature_5.png

assets/skins/nature_6.png

assets/skins/nature_7.png

assets/skins/nature_8.png

assets/skins/nature_9.png

icons/difficulty_easy.svg

icons/difficulty_hard.svg

icons/difficulty_medium.svg

icons/size_2.svg

icons/size_3.svg

icons/skin_default.svg

icons/skin_nature.svg

icons/skins/default/1.svg

icons/skins/default/2.svg

icons/skins/default/3.svg

icons/skins/default/4.svg

icons/skins/default/5.svg

icons/skins/default/6.svg