itch.io

You can start with making bare boxes with a font. So people can make custom buttons and menu box.

For icons, The most important ones are settings restart, play, pause, stop, option (three horizontal lines) and share. 

You do have a home icon, but if it is used as such, then it cannot be used as a store icon.

You can also look at some android games, which I supposed this is targeting. For example, an i button might be useful there.