Here is an instruction how you can add a custom icons to your iPhone/iPad:
Open the Shortcuts app on your iPhone / iPad
Tap on + on the top right to add a new shortcut
Tap the + Add Action button in the new shortcut
Tap the 'Scripting' option
Tap the 'Open App' option
Choose the app you want to customise with new icon
Tap on the ellipses icon in the blue circle (below 'Next' button)
Tap on 'Add to Home Screen' button
Add the name of the app and tap the default icon > Choose Photo
Choose your icon from the Limitless icon pack
Tap 'Add' on the top right of your screen
Your new app icon will be added to your device.
I know this feels like a lot of steps but it's very easy to run through this in a few seconds.
Also when opened, it will first redirect to Shortcuts app and then open the actual configured app. There is no way to get around this in iOS at the moment. But yeah the interface does look different and fun.
App shortcuts briefly open the shortcuts app first (UPD: In iOS 14.3 Beta 2 this will be removed)