Plesk backup finishes with the warning message in Plesk Obsidian:
Warning: server "server"
Unable to dump theme skin [Error in execute 'branding_theme' ' --pack -name heavy-metal -destination "C:Program Files (x86)ParallelsPleskPrivateTemppmm-tsd-dts-dest-b3d187e64a7541c79666ec0e17515cb7"': Skin with name heavy-metal was not found on filesystem.
#PPPM-10883 "It is now possible to create a backup without a warning if a folder that contained a custom branding theme was removed."
- Plesk Obsidian 25 September 2019 (fixed as PPP-43734)
If update is not possible for some reason you may try the following
The warning can be safely ignored.