gnome-calculator-42.0, gnome-system-monitor-42.0 and the like (*) do not change their UI theme when toggling "Light/Dark" in gnome-control-center-42.0's "Appearance" section.
Observation: they do when xdg-desktop-portal-kde is not running.
An #gentoo-desktop discussion pointed out that other xdg-desktop-portal-* packages are missing or outdated from the tree.
Not fully well knowing how to test these, I will still create a PR for these ebuilds anyway for further discussion.
(*) I don't know what exactly makes applications support this toggle. I thought it was dependence on libadwaita, but not sure.