GtkCalendar has some odd issues with top padding, which can be illustrated by adding some padding to the whole widget.


calendar {
 padding: 3px;
}


To fix this and get a square shaped background on the selected date (as would be the generally expected result of the above code) I wrote an ugly hack, using box-shadow:

calendar {
  padding: 9px 3px 3px 3px;
}

calendar:selected {
  background: #000000;
  box-shadow: 0 -6px #000;
}


I would say this has to be fixed in GtkCalendar itself. Subsequently some more padding should be added to Adwaita/HighContrast or even GtkCalendar itself after all. After all the current default padding looks bad/crammed with pretty much any theme...