Sorry to hear you are having issues. I am using Fences V126.96.36.199 and I can edit individual background color without any issue. I am also running on Windows 10 v1909 (OS Build 18363.778).
Need to know,
1) Windows 10 full version numbers, please use Winver.exe for it.
2) Fences version.
3) And Please try the purge and reinstall steps as guided here : https://forums.stardock.com/486084/fences-support-faq#reinstalling
Just to be sure, re-download the latest version from your account. Reboot after the purge, reinstall, reboot again. Retest and report back here.
Stardock Community Assistant.