Now everything is working properly again.
The one described by philm here under point 3:
sudo pacman -Rdd xdg-desktop-portal-gnome
sudo pacman -S xdg-desktop-portal-gtk
is not enough. I also uninstalled the package xdg-desktop-portal:
sudo pacman -Rdd xdg-desktop-portal
So even in all Chrome-based browsers, the Internet pages are automatically called up again in dark if the page supports it.
On my system with Cinnamon Desktop only the package “xdg-desktop-portal-gtk” is installed now.