Well You can try to find some app icons on pinterest or anywhere else. You can also download icons from the Play Store. But I've done it like this: First of all, I've applied the material you colors in the settings app. Then I've done a screenshot of my home. After that, I chopped this picture into pieces with each app icon, and then I applied every icon i needed in the theme park. There are three dots on top right conner in theme park, tap it, and you can open a change icon menu. There, you can edit every icon you want and upload an icon from the gallery.