Add transparency to skin images, fix background color

android/gradle.properties

assets/icons/difficulty_easy.png

assets/icons/difficulty_hard.png

assets/icons/difficulty_medium.png

assets/icons/size_2.png

assets/icons/size_3.png

assets/icons/skin_default.png

assets/icons/skin_nature.png

assets/skins/default_1.png

assets/skins/default_2.png

assets/skins/default_3.png

assets/skins/default_4.png

assets/skins/default_5.png

assets/skins/default_6.png

assets/skins/default_7.png

assets/skins/default_8.png

assets/skins/default_9.png

assets/skins/empty.png

assets/skins/nature_1.png

assets/skins/nature_2.png