You may want to add the select2 script to the select boxes. First, change the combobox script to select2. In WP Admin -> Search & Filter -> Settings, look for the Combobox script and set that to Select2. Here is a screenshot:
https://www.screencast.com/t/g58SM1O9
Then you need to add this JavaScript to the page:
<script>
(function ( $ ) {
$(document).on("sf:ajaxfinish", ".searchandfilter", function(){
$('select.sf-input-select').select2();
});
}(jQuery));
jQuery(document).ready(function($){
$('select.sf-input-select').select2();
});
</script>
You can place this inside a text widget on the page, put the code in an external JavaScript file (enqueued from your child theme functions.php file), or use a plugin to add it, such as:
https://wordpress.org/plugins/custom-css-js/
(note that this plugin does not need the script tags).
Then you need to modify the Select2 CSS. This post shows the complete Select2 CSS that we already include in our plugin:
https://support.searchandfilter.com/forums/topic/customise-dropdown-combobox/#post-180345
At the end is an example of how to modify it.
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