style.css

.flatpickr-monthSelect-months {
  margin: 10px 1px 3px 1px;
  flex-wrap: wrap;
}

.flatpickr-monthSelect-month {
  background: none;
  border: 0;
  border-radius: 2px;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  color: #393939;
  cursor: pointer;
  display: inline-block;
  font-weight: 400;
  margin: 0.5px;
  justify-content: center;
  padding: 10px;
  position: relative;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  text-align: center;
  width: 33%;
}

.flatpickr-monthSelect-month.disabled {
	color: #eee;
}

.flatpickr-monthSelect-month.disabled:hover,
.flatpickr-monthSelect-month.disabled:focus {
	cursor: not-allowed;
	background: none !important;
}

.flatpickr-monthSelect-theme-dark {
  background: #3f4458;
}

.flatpickr-monthSelect-theme-dark .flatpickr-current-month input.cur-year {
  color: #fff;
}

.flatpickr-monthSelect-theme-dark .flatpickr-months .flatpickr-prev-month,
.flatpickr-monthSelect-theme-dark .flatpickr-months .flatpickr-next-month {
  color: #fff;
  fill: #fff;
}

.flatpickr-monthSelect-theme-dark .flatpickr-monthSelect-month {
  color: rgba(255, 255, 255, 0.95);
}

.flatpickr-monthSelect-month:hover,
.flatpickr-monthSelect-month:focus {
  background: #e6e6e6;
  cursor: pointer;
  outline: 0;
}

.flatpickr-monthSelect-theme-dark .flatpickr-monthSelect-month:hover,
.flatpickr-monthSelect-theme-dark .flatpickr-monthSelect-month:focus {
  background: #646c8c;
  border-color: #646c8c;
}

.flatpickr-monthSelect-month.selected {
  background-color: #569ff7;
  color: #fff;
}

.flatpickr-monthSelect-theme-dark .flatpickr-monthSelect-month.selected {
  background: #80cbc4;
  -webkit-box-shadow: none;
  box-shadow: none;
  color: #fff;
  border-color: #80cbc4;
}