Skins made for the iOS emulator GBA4iOS. Not affiliated with the developers of the app.
Open in Safari on iOS device, wait for download to complete, choose "Open in GBA4iOS" and it should then appear in your Skin selection.
Development on these skins has ceased, since the development of the app has ceased. The developer is working on a new app called Delta which I may port these over to if custom skins are supported.