XPasscode is a fully themeable "engine" for users to create their own passcodes. It is the successor of Piano passcode but since version 2.0 they have largely the same codebase.
You can read more about XPasscode here.
For a video that showcases some of the default themes you can go here. Subreddit can be found at /r/xpasscode and it includes links to the documentation for creating your own themes.