Custom Styling

Anyone?

That is based on Bootstrap theme, your theme need to be using Bootstrap CSS for that to work.

If your theme already uses it, this is the code I used in my theme

if(function_exists('wp_pagenavi')) {
        wp_pagenavi( [
            'before' => '<nav class="pagination pagination-sm visible-lg clearfix">',
            'after'  => '</nav>'
        ] );
    }

and here is the CSS

/* WP-PageNavi */
.pagination .wp-pagenavi a,
.pagination .wp-pagenavi span {
    float: left;
    padding: 5px 10px;
    text-decoration: none;
    border: 1px solid #ccc;
    border-left-width: 0;
}
.pagination .wp-pagenavi .pages {
    border-left-width: 1px;
}
.pagination .wp-pagenavi .current {
    color: #999999;
    background-color: #f5f5f5;
}