The one I edited was Big Sur day 2, yours was on Big Sur, maybe you can try delete both of that theme. Redownload it again from that link you gave me. Apply Big Sur Day 2 and do the Edit as I high lighted above. Click apply button and if it don't work than something else is not working.
Maybe try purge and reinstall Curtains as mentioned here : https://forums.stardock.com/500728/curtains-support-faq#CurtainsPurge Just make sure to re-download the latest version from your account. Reboot after the purged. Reinstall, and Reboot again after the installation. Retest it. Report back here.
Stardock Community Assistant