Add a CSS class to the Sort facet’s “select” element

<?php
// With JS:
add_action( 'facetwp_scripts', function() {
?>
    <script>
      document.addEventListener('facetwp-loaded', function() {
        fUtil('.facetwp-type-sort select').addClass('form-select');
      });
    </script>
<?php
}, 100 );

// Or, with PHP:
add_filter( 'facetwp_facet_html', function( $output, $params ) {
  if ( 'sort' == $params['facet']['type'] ) { //
    $output = str_replace( 'select', 'select class="form-select"', $output );
  }
  return $output;
}, 10, 2 );