Here is another idea with only CSS.
$(".datepicker").datepicker();
.table-condensed > thead > tr:nth-child(2) > .prev {
font-size: 0;
background: url("data:image/svg+xml,%0A%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round' %3E%3Cline x1='19' y1='12' x2='5' y2='12'%3E%3C/line%3E%3Cpolyline points='12 19 5 12 12 5'%3E%3C/polyline%3E%3C/svg%3E") 50% 50% / contain no-repeat;
}
.table-condensed > thead > tr:nth-child(2) > .next {
font-size: 0;
background: url("data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round' %3E%3Cline x1='5' y1='12' x2='19' y2='12'%3E%3C/line%3E%3Cpolyline points='12 5 19 12 12 19'%3E%3C/polyline%3E%3C/svg%3E") 50% 50% / contain no-repeat;
}
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.9.0/css/bootstrap-datepicker.min.css">
<input type="text" class="datepicker">
<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.9.0/js/bootstrap-datepicker.min.js"></script>
Change the color of icons by using mask
: (You can already change the color of SVG by changing the fill or stroke color but let's say you're using a PNG instead of SVG)
$(".datepicker").datepicker();
.table-condensed > thead > tr:nth-child(2) > .prev {
font-size: 0;
--mask: url("data:image/svg+xml,%0A%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round' %3E%3Cline x1='19' y1='12' x2='5' y2='12'%3E%3C/line%3E%3Cpolyline points='12 19 5 12 12 5'%3E%3C/polyline%3E%3C/svg%3E") 50% 50% / contain no-repeat;
-webkit-mask: var(--mask);
mask: var(--mask);
background: red;
}
.table-condensed > thead > tr:nth-child(2) > .next {
font-size: 0;
--mask: url("data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' viewBox='0 0 24 24' fill='none' stroke='currentColor' stroke-width='2' stroke-linecap='round' stroke-linejoin='round' %3E%3Cline x1='5' y1='12' x2='19' y2='12'%3E%3C/line%3E%3Cpolyline points='12 5 19 12 12 19'%3E%3C/polyline%3E%3C/svg%3E") 50% 50% / contain no-repeat;
-webkit-mask: var(--mask);
mask: var(--mask);
background: blue;
}
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.9.0/css/bootstrap-datepicker.min.css">
<input type="text" class="datepicker">
<script src="https://code.jquery.com/jquery-3.3.1.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.9.0/js/bootstrap-datepicker.min.js"></script>