discourse/app/assets/stylesheets/common/components/date-picker.scss

.pika-table {
  th, td {
    padding: 0 !important;
    border-top: none !important;
    border-bottom: none !important;
  }
}

.date-picker-wrapper {
  position: relative;
  display: inline-block;
}

.date-picker {
  text-align: center;
  width: 80px;
  margin: 0;
}

.pika-single {
  position: absolute !important;
}