The issue in 91.3.1 was bug 1741095 which is fixed in version 91.3.2.
I can't reproduce your Calendar issue using 91.3.2 on Fedora 34 Workstation, Gnome 40 desktop with the light Adwaita operating system theme and the Thunderbird Dark theme, or using the Fedora supplied 91.2.0.
The issue in 91.3.1 was bug 1741095 which is fixed in version 91.3.2.
I can't reproduce your Calendar issue using 91.3.2 on Fedora 34 Workstation, Gnome 40 desktop with the light Adwaita operating system theme and the Thunderbird Dark theme, or using the Fedora supplied 91.2.0.