Enable Web Inspector on iPhone. Settings > Safari > Advanced > Enable Web Inspector
Connect your phone to your mac
Open wefwef.app on the phone, connect to it from macos safari developer menu.
Change the icon urls in the inspector then install the app on the phone.
You can replace the icon url by searching for "apple-touch-icon" and changing the href property everywhere you see it to this: https://i.imgur.com/kUxXoIA.png