It’s not, I loaded it last night and it didn’t work for me as well. Now what you can do is copy and paste the theme.yaml into your theme folder and produce the same effect.
The backgrounds are downloadable from his GitHub
This will add the background as long as your pic is stored in the reference folder.
lovelace-background: center / cover no-repeat fixed url('/local/pics/red-blue.jpeg')
www=local and I created the folder pic
# From https://github.com/basnijholt/lovelace-ios-themes
#
# iOS Light Mode Theme - dark-green
#
ios-light-mode-dark-green-alternative:
# Global
lovelace-background: center / cover no-repeat fixed url('/local/pics/red-blue.jpeg')
primary-color: "#FF9F09" # from Apple systemOrange dark mode
light-primary-color: "#B6B6C1" # light gray (icons on left menu)
primary-background-color: "#2c2c2e" # systemGray5 dark mode
secondary-background-color: rgba(25, 25, 25, 0.7)
divider-color: "#98989d" # from Apple systemGray dark mode
accent-color: rgba(255, 159, 9, 1)
....