calendar: Add css style for marked days

As part of a6f9052cf1, marked days lost
their style, essentially making that function worthless.  Previously,
they were simply bolded, but that doesn't give them proper justice.

gtk/theme/Default/_common.scss

@@ -3510,6 +3510,10 @@ calendar {
         @extend %selected_items;
         border-radius: 3px;
       }
+
+      &:checked {
+        background-color: gtkalpha($selected_bg_color, 0.3);
+      }
     }
 
     > label.day-number.other-month {