I just found out that it actually is possible if you have a Mac, although a bit cumbersome.
This Post explains it:
- 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
So change this:
<link rel="apple-touch-icon" type="image/png" sizes="180x180" href="/favicons/apple-touch-icon-180.png">
to this:
<link rel="apple-touch-icon" type="image/png" sizes="180x180" href="https://i.imgur.com/kUxXoIA.png">
And do this for all the "apple-touch-icon"s.