Improve theme, redraw icons

android/gradle.properties

assets/icons/difficulty_easy.png

assets/icons/difficulty_hard.png

assets/icons/difficulty_medium.png

assets/icons/difficulty_nightmare.png

assets/icons/size_10x10.png

assets/icons/size_15x15.png

assets/icons/size_20x20.png

fastlane/metadata/android/en-US/changelogs/15.txt

fastlane/metadata/android/fr-FR/changelogs/15.txt

icons/difficulty_easy.svg

icons/difficulty_hard.svg

icons/difficulty_medium.svg

icons/difficulty_nightmare.svg

icons/size_10x10.svg

icons/size_15x15.svg

icons/size_20x20.svg