one-dark/gtk-3.0/sass/ui/calendar.scss

//
// Calendar
//
calendar {
  color: $fg_color;
  border: 0;
  border-radius: 0;
  padding:6px 4px;

  &:selected {
    background-color: $selected_bg_color;
    color: $selected_fg_color;
    border-radius: 50%;
  }
  &.header {
    color: $fg_color;
    border: none;
    border-radius: 0;
  }
  &.button,
  &.button:focus {
    color: $highlight_color;

    @include button(undecorated);

    &:hover {
      color: $fg_color;
    }
    &:disabled {
      color: $insensitive_fg_color;
      background-color: transparent;
      background-image: none;
    }
  }
  &:indeterminate {
    color: gtkalpha(currentColor, 0.55);
  }
  &.highlight {
    color: $error_color;
  }
}