I loved the artwork on the invite, with colors possibly referring to the purported new colors for the iPhone XR and maybe the return of the rainbow logo. I quickly whipped up these wallpapers below for you to download for free.
Apple September Special Event Wallpaper for iPhone X, iPhone XR, iPhone XS, and iPhone XS Max
Note: The original Apple invite (and hence the wallpaper above) uses a light background, so for users with iPhone X, iPhone XS and iPhone XS Max devices, do keep an eye on your battery life.
Apple September Special Event Wallpaper for iPad, iPad Pro, and iPad Air