There's a hidden surprise in the iOS app that allows you to change the app's icon. To find it, scroll up until you see Today and keep scrolling up in a Journal project until the icons' screen shows up.
From there, you can select any icon that's available to you. The more snippets you add, the more icons you'll unlock!
To reset the app's icon to the original one, open the icons' screen and tap on "Reset" at the top right.
Tip: You might have to restart your phone to see the icon change on your phone's home screen.