Cannot select window color theme after upgrade from 18.04 to 20.04
After upgrading Ubuntu from version 18.04 LTS to 20.04 LTS some icons have not changed to the new version and window color themes are not selectable.
I saw the problem too, after upgrade from 18.04 to 20.04.
Installing the Yaru theme packages can fix it.
sudo apt install yaru-theme-gtk yaru-theme-sound yaru-theme-gnome-shell yaru-theme-icon yaru-theme-unity