Custom App Icon for mods

You can definitely create your own extension that uses a custom app icon and it will work no problem when you "Load unpacked" it in developer mode at the URL opera://extensions.

As for uploading said extension to https://store.gx.me/mods, I'm not sure if there are any restrictions for what studios are allowed to do app icons there or if there are any restrictions for what mods can be tagged with the app-icon tag. For example, I think the mods-2-0 tag is restricted to Opera-provided mods but there are still lots of mod 2.0 mods that aren't tagged as such.

You can keep installing mods form other studios and check at the URL opera://configure/mods/interface/app_icon to see if they an app icon. At https://store.gx.me/mods/, maybe sort by last modified so you get newer extensions. You'll still have to skip Opera studio mods though.

You can create a folder named "test", put:

manifest.json:

{
  "mod" : {
    "schema_version" : 2,
    "payload" : {
      "app_icon" : [ {
            "id" : "app_icon_01",
            "name" : "Test",
            "path" : "app_icon1.png"
        }
      ]
    }
  },
  "manifest_version" : 3,
  "name" : "Test",
  "icons" : {
    "512" : "icon_512.png"
  },
  "version" : "1.2.1"
}

in it, put icon_512.png and app_icon_01.png (290px x 290px) in it, goto the URL opera://extensions, turn on developer mode, click "Load unpacked" and point it to the test folder to load the extension.

Then, you can goto the URL opera://configure/mods/interface/app_icon, set the mod's icon and restart Opera.

Then, Opera's pinned taskbar icon should be that custom icon.