jquery-ui-datepicker-bootstrap4.css

/* Based on: https://gist.github.com/miwahall/7028640 */

.ui-datepicker {
  background-color: #fff;
  border: 1px solid #66AFE9;
  border-radius: 4px;
  box-shadow: 0 0 8px rgba(102, 175, 233, .6);
  display: none;
  margin-top: 4px;
  padding: 10px;
  width: 240px;
}

.ui-datepicker a,
.ui-datepicker a:hover {
  text-decoration: none;
}

.ui-datepicker a:hover,
.ui-datepicker td:hover a {
  color: #2A6496;
  -webkit-transition: color 0.1s ease-in-out;
  -moz-transition: color 0.1s ease-in-out;
  -o-transition: color 0.1s ease-in-out;
  transition: color 0.1s ease-in-out;
}

.ui-datepicker .ui-datepicker-header {
  margin-bottom: 4px;
  text-align: center;
}

.ui-datepicker .ui-datepicker-title {
  font-weight: 700;
}

.ui-datepicker .ui-datepicker-prev,
.ui-datepicker .ui-datepicker-next {
  cursor: pointer;
  font-family: 'Glyphicons Halflings';
  -webkit-font-smoothing: antialiased;
  font-style: normal;
  font-weight: normal;
  height: 20px;
  line-height: 1;
  margin-top: 2px;
  width: 30px;
}

.ui-datepicker .ui-datepicker-prev {
  float: left;
  text-align: left;
}

.ui-datepicker .ui-datepicker-next {
  float: right;
  text-align: right;
}

.ui-datepicker .ui-icon {
  display: none;
}

.ui-datepicker .ui-datepicker-calendar {
  table-layout: fixed;
  width: 100%;
}

.ui-datepicker .ui-datepicker-calendar th,
.ui-datepicker .ui-datepicker-calendar td {
  text-align: center;
  padding: 4px 0;
}

.ui-datepicker .ui-datepicker-calendar td {
  border-radius: 4px;
  -webkit-transition: background-color 0.1s ease-in-out, color 0.1s ease-in-out;
  -moz-transition: background-color 0.1s ease-in-out, color 0.1s ease-in-out;
  -o-transition: background-color 0.1s ease-in-out, color 0.1s ease-in-out;
  transition: background-color 0.1s ease-in-out, color 0.1s ease-in-out;
}

.ui-datepicker .ui-datepicker-calendar td:hover {
  background-color: #eee;
  cursor: pointer;
}

.ui-datepicker .ui-datepicker-calendar td a {
  text-decoration: none;
}

.ui-datepicker .ui-datepicker-current-day {
  background-color: #4289cc;
}

.ui-datepicker .ui-datepicker-current-day a {
  color: #fff
}


.ui-datepicker .ui-datepicker-calendar .ui-datepicker-unselectable {
  background-color: #eff0f1;
  cursor: default;
}

.ui-datepicker .ui-datepicker-calendar .ui-datepicker-unselectable:hover {
  background-color: #eff0f1;
  cursor: default;
}

/* Fix icons (Here I am using Font Awesome 4.7 - Replace with whatever you prefer) */
.ui-datepicker-prev span {
  display: none;
}

.ui-datepicker-prev:before {
  display: inline-block !important;
  font: normal normal normal 14px/1 FontAwesome !important;
  font-size: 14px !important;
  font-size: inherit !important;
  text-rendering: auto !important;
  -webkit-font-smoothing: antialiased !important;
  -moz-osx-font-smoothing: grayscale !important;
  content: '\f060';
}

.ui-datepicker-next span {
  display: none;
}

.ui-datepicker-next:after {
  display: inline-block !important;
  font: normal normal normal 14px/1 FontAwesome !important;
  font-size: 14px !important;
  font-size: inherit !important;
  text-rendering: auto !important;
  -webkit-font-smoothing: antialiased !important;
  -moz-osx-font-smoothing: grayscale !important;
  content: '\f061';
}
/* END: Fix icons */

Note: For Font Awesome 5 Pro, you'll use the following:

/* Fix icons */
.ui-datepicker-prev span {
    display: none;
}

.ui-datepicker-prev:before {
    font-family: 'Font Awesome 5 Pro';
    font-weight: 300;
    content: '\f060';
}

.ui-datepicker-next span {
    display: none;
}

.ui-datepicker-next:after {
    font-family: 'Font Awesome 5 Pro';
    font-weight: 300;
    content: '\f061';
}
/* END: Fix icons */

And here's a JSFiddle demo: https://jsfiddle.net/567yka9m/1/