Light and Dark. Choose wisely.

iOS Custom App Icons

Create a new shortcut in the Shortcut App

Add a script action (Open App)

Select the app you want to open and click the share icon on the bottom. Select Add to Home Screen.

Choose now the image from gallery or file. (It depends on where you saved the icons you downloaded!) And select the desired icon.

Give a name to the shortcut and you are done! Bonus Tip: leave the shortcut name blank to have an icon without the name label on the home screen!