This is an issue with the third-party GNOME Shell theme (not the GTK or libadwaita theme) being used here.
Because third-party GNOME Shell themes are layered on top of the default GNOME Shell theme used by the session (in this case, the Zorin OS GNOME Shell theme), elements of the default theme may still appear if they aren't explicitly overwritten by the third-party theme developer.
To resolve this issue, the developer of this third-party GNOME Shell theme would need to add the background-gradient-direction: none; CSS variable in the .calendar-today and .quick-toggle-menu .header.active elements.