The current CSS for the libhandy viewswitcher no longer works with libhandy1 (it only works with libhandy0.) It works properly on KDE Neon because Ubuntu 20.04 only ships libhandy0, but Arch Linux and Ubuntu 20.10 both ship libhandy1 and newer application versions that use it (such as GNOME Clocks 3.38.)
To test on KDE Neon, download and install the following 20.10 .deb packages, then completely close the libhandy demo and GNOME Clocks before re-opening them (using handy-1-demo instead of handy-0.0-demo):
Since this isn't removing or replacing the older CSS, libhandy0 applications are still supported. I found that I needed more lines to achieve roughly the same style as the current libhandy0 CSS (I referenced upstream Adwaita to find some of the selectors.)
Differences between the current libhandy0 styling and the new libhandy1 styling:
Suggestions/revisions are welcome to take care of the discrepancies and simplify this if possible.